Absorbent article comprising a modified water absorbent resin

ABSTRACT

An absorbent member and a method for making such an absorbent member. The absorbent member includes a modified water absorbent resin having good production efficiency, absorbency against pressure, absorption speed, gel strength, and liquid permeability. The modified water absorbent resin may be made by mixing a water absorbent resin, water, and a water-soluble radical polymerization initiator without addition of an ethylenically unsaturated monomer to obtain a water absorbent resin composition, and irradiating the water absorbent resin composition with active energy rays. During irradiation, the surface water content of the water absorbent resin is at least 3.0 wt %.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/732,589, filed Apr. 4, 2007 now U.S. Pat. No. 7,745,507.

FIELD OF THE INVENTION

This invention relates to absorbent articles, especially diapers andtraining pants, which include an absorbent member and a modified waterabsorbent resin. Specifically, an absorbent article including a waterabsorbent resin that is surface cross-linked while maintain a particularsurface water content of the resin particles.

BACKGROUND OF THE INVENTION

A water absorbent resin has been hitherto used as a component forhygienic materials such as sanitary cotton, disposable diaper, andabsorbents for other kinds of body fluid. As typical examples of thewater absorbent resin, hydrolyzate of starch-acrylonitrile graftpolymer, neutralized starch-acrylic acid graft polymer, saponified vinylacetate-acrylic ester copolymer, hydrolyzate of acrylonitrile copolymeror acrylamide copolymer, and cross-linked product thereof, and partiallyneutralized cross-linked acrylic acid may be cited. These waterabsorbent resins have an internal cross-linked structure and arein-soluble in water. The characteristics of such a water absorbent resininclude for example high absorption capacity, high absorption speed,high gel strength, and fully satisfactory suction power necessary forsucking water from a medium. The water absorbing properties are affectedby cross-link density, and an increase in the cross-link densitytypically leads to an increase in the gel strength but a decrease in theamount of water absorbed. Particularly, increased absorption capacitytypically leads to reduced absorption speed, reduced gel strength, andreduced suction power, for example. The water absorbent resin havingimproved absorption capacity, therefore, would possibly induceinhomogeneous absorption of water and lead to aggregation of absorbentparticles when the water absorbent resin particles contact with water,and also induce dramatic decrease in absorption speed because the wateris not diffused throughout the entire volumes of water absorbent resinparticles.

For obtaining a water absorbent resin having high absorption capacityand a comparatively satisfactory absorption speed, a method for coatinga surface of water absorbent resin particles with a surfactant or anonvolatile hydrocarbon has been available. This method indeed can exaltthe dispersibility of initially absorbed water but does not havesufficient effects on enhancing absorption speed and suction power ofindividual resin particles.

As a means to produce a polyacrylic acid based water absorbent polymerhaving improved water absorbing properties, a method which comprisesheating an aqueous composition of a polymer having a partial alkalimetal salt of polyacrylic acid as a main component and having a lowcross-link density in the presence of a water-soluble peroxide radicalinitiating agent thereby introducing a cross-link therein by radicalcross-linking has been proposed (U.S. Pat. No. 4,910,250). It isdifficult to distribute uniformly internal cross-links in the polymerand uneasy to adjust the cross-link density. Accordingly, a polymerwhich contains water-soluble polyacrylic acid gel having low cross-linkdensity is obtained and then the polymer is heated together with apersulfate added thereto as a polymerization initiator. U.S. Pat. No.4,910,250 states that excellent water absorbing properties can beattained and a water absorbent resin having no stickiness can beobtained because the adjustment of the amount of the initiator to beadded can allow precise control of cross-link density and uniformpresence of cross-link in the polymer.

While the persulfate which is used in the U.S. Pat. No. 4,910,250 isdecomposed by heat, it is also decomposed by ultraviolet rays togenerate radicals (J. Phys. Chem., 1975, 79, 2693, J. Photochem.Photobiol., A. 1988, 44, 243). Since the persulfate acts as apolymerization initiator, the aqueous solution of a water-soluble vinylmonomer, when exposed to radiation, undergoes polymerization and radicalcross-linking simultaneously (JP-A 2004-99,789). A reaction system hasalso been known, which comprises adding a hydrophilic polymer componentand a photo-polymerization initiator, further adding a cross-linkingagent thereto, and irradiating them with ultraviolet rays to form aninternal cross-link (WO 2004/031253).

Further, a method which comprises treating a surface of a waterabsorbent resin to increase cross-link density of the surface of waterabsorbent resin has also been known (U.S. Pat. Nos. 4,666,983 and5,422,405, for example). Such water absorbent resins as cited in thepreceding publications comprise a reactive functional group on theirsurfaces. By adding a surface cross-linking agent capable of reactingwith the functional groups in order to introduce cross-links between thefunctional groups, cross-link density on the surface of water absorbentresin can be increased and a water absorbent resin having excellentwater absorbing properties even under pressure can be obtained.

Further, since the use of the surface cross-linking agent requires ahigh temperature and a long time for the reaction of forming cross-linkand entails the problem of suffering persistence of unalteredcross-linking agent, a method which comprises contacting an aqueoussolution containing a peroxide radical initiating agent with a resin,heating the resin to decompose the radical initiating agent andintroduce cross-links into polymer molecular chains in the neighborhoodof the surface of the resin has been proposed (U.S. Pat. No. 4,783,510).In the working example, a water absorbent resin exhibiting exaltedabsorption capacity was obtained by heating with superheated steam at130° C. for 6 minutes.

SUMMARY OF THE INVENTION

The present invention refers to an absorbent member for use in absorbentarticles, the absorbent member comprising modified water absorbentresin. The water absorbent resin is made by a method comprising:

-   -   (i) a mixing step comprising mixing water absorbent resin,        water, and a water-soluble or a heat-degradable radical        polymerization initiator without addition of an ethylenically        unsaturated monomer, to obtain a water absorbent resin        composition; and    -   (ii) an irradiating step comprising irradiating said water        absorbent resin composition obtained in the mixing step with        active energy rays,        wherein the surface water content of said water absorbent resin        in said water absorbent resin composition at least at any point        of time in the irradiating step (ii) is controlled to a level of        not lower than 3.0% by weight based on 100% by weight of the        water absorbent resin.

Preferably, the amount of water mixed in step (i) exceeds 20 parts byweight and is not more than 100 parts by weight based on 100 parts byweight of the water absorbent resin.

The present invention further refers to a method of making suchabsorbent members and modifications of making the absorbent members ofclaims 1 to 3 as set out in the following description and in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic diagram of a measuring device to be used indetermining the saline flow conductivity (SFC).

DETAILED DESCRIPTION OF THE INVENTION

The object of surface cross-linking a water absorbent resin is achievedby a method for producing a water absorbent resin having an improvedbalance between absorption capacity and absorption speed. Generally,this object requires a cross-linking agent having at least twofunctional groups capable of reacting with the functional group presenton the surface of the water absorbent resin. As typical examples of thecross-linking agent, polyhydric alcohols, polyvalent glycidyl ethers,haloepoxy compounds, polyvalent aldehydes, polyvalent amines, andpolyvalent metal salts may be cited. Since the cross-linking agent haslow reactivity, the relevant reaction is required to be carried out atan elevated temperature and occasionally to be retained in a heatedstate for a long time. The reaction, therefore, requires relatively highamounts of energy and time.

This invention aims at providing a method for an efficient production ofa water absorbent resin having good absorbency against pressure,absorption speed, gel strength, and permeability of liquid.

It has been found that surfaces of the particles need to retain water tosome extent in order to effectively introduce a cross-linking structureon surfaces of water absorbent resin particles. Namely, it has beenfound that in conventional methods, when surfaces of the particles arenot humid to some extent, introduction of a cross-linking structure onsurfaces of particles cannot effectively be attained, and therefore awater absorbent resin having good water absorbent properties cannot beproduced. Based on this finding, the present inventors have attempted toconduct surface treatment of water absorbent resin particles being humidto some extent. In addition, the present inventors have also found thatsurface cross-linking efficiency and water absorbent properties of theresultant water absorbent resin can be improved, by irradiation withactive energy rays without using conventional addition of a surfacecross-linking agent. Namely, herein a method for the production of amodified water absorbent resin is provided, which comprises (i) a mixingstep comprising mixing a water absorbent resin, water, and awater-soluble radical polymerization initiator without addition of anethylenically unsaturated monomer, to obtain a water absorbent resincomposition, and (ii) an irradiating step comprising irradiating saidwater absorbent resin composition obtained in the mixing step withactive energy rays, wherein the surface water content of said waterabsorbent resin in said water absorbent resin composition at least atany point of time during the irradiating step (ii) is controlled to alevel of not lower than 3.0% by weight based on 100% by weight of thewater absorbent resin. Preferably, the amount of water mixed in step (i)exceeds 20 parts by weight and is not more than 100 parts by weightbased on 100 parts by weight of the water absorbent resin.

According to the method of this invention, a uniform cross-linkingstructure can be introduced on a surface of water absorbent particles.As a result, the obtained water absorbent resin has good absorptioncapacity, good absorption speed, good gel strength, and good suctionpower.

Since the method of this invention for the production of a modifiedwater absorbent resin achieves surface cross-linking by irradiation withactive energy rays, the water absorbent resin can be modified in ashorter period as compared with the conventional method.

The method for the production of a modified water absorbent resincomprised in the absorbent members according to this invention will bedescribed in detail below.

(a) Water Absorbent Resin

The water absorbent resin which can be used in this invention is awater-swellable, water-insoluble, cross-linked polymer which can form ahydrogel. The term “ability to swell in water” as used in this inventionrefers to the free swelling capacity of a given sample in an aqueous0.9% by weight sodium chloride solution (physiological saline), i.e. theability of the sample to absorb the physiological saline essentially notlower than 2 g/g and preferably in the range of from 5 to 100 g/g andmore preferably in the range of from 10 to 60 g/g. The term “insolublein water” as used herein means that an uncross-linked water-solublecomponent (a water-soluble polymer; hereinafter also called as “anelutable and soluble portion”) in the water absorbent resin, which ispreferably in the range of from 0 to 50% by weight, more preferably notmore than 25% by weight, still more preferably not more than 15% byweight, and particularly preferably not more than 10% by weight. In thisconnection, as a value of centrifuge retention capacity, a valuemeasured by the method specified in the working example cited below isadopted. And as a value of an elutable and soluble portion, a valuemeasured by a method described below is adopted.

Method for Measuring an Elutable and Soluble Portion

In a covered plastic container (with a diameter of 6 cm and a height of9 cm) having a volume of 250 ml, 184.3 g of physiological saline isplaced, 1.00 g of water absorbent resin is added. An elutable andsoluble portion in a resin is extracted by stirring the mixture for 16hours with a magnetic stirrer having a diameter of 8 mm and a length of25 mm at a rotation speed of 500 rpm. This extract is filtrated with onesheet of a filter paper (0.26 mm in thickness and 5 μm in retainedparticle diameter; made by Advantec Toyo K.K. and sold under the productname of “JIS P 3801 No. 2”). Then, 50.0 g of the resultant filtrate istaken to make a solution for measuring.

First, only physiological saline is titrated with 0.1 N of an aqueoussolution of sodium hydroxide to pH 10. Then, it is titrated with 1 N ofan aqueous solution of hydrochloric acid to pH 2.7, to obtaincomparative (blank) titration amounts (called as [bNaOH] and [bHCl],respectively).

By conducting the same operation of titration with the solutions formeasuring, titration amounts (called as [NaOH] and [HCl], respectively)were obtained.

For instance, in the case of a water absorbent resin consisting of knownamounts of acrylic acid and sodium salt thereof, an elutable and solubleportion in the water absorbent resin can be calculated on the basis ofan average molecular weight of the monomers and the titration amountsobtained by the above described operation in accordance to the equationdescribed below. In the case of an unknown amount, an average molecularweight of a monomer is calculated, by using a neutralization ratioobtained by titration according to the equation described below.Elutable and soluble portion (% by weight)=0.1×(Average molecular weightof monomer)×184.3×100×([HCl]-[bHCl])/1000/1.0/50.0Neutralization ratio (% by mol)=[1−([NaOH]−[bNaOH])/([HCl]−[bHCl])]×100

As used herein, the term “modification” refers to all physical orchemical actions performed on a given water absorbent resin, includingsurface cross-linking, formation of pores therein, and impartinghydrophilic or hydrophobic property thereto, for example.

The water absorbent resin which can be used in this invention is notparticularly restricted but is only required to be capable of beingobtained by polymerizing a monomer component essentially containing anethylenically unsaturated monomer by means of any of the known methods.

The ethylenically unsaturated monomer is not particularly restricted butis preferred to be a monomer having an unsaturated double bond at theterminal thereof. Typical examples thereof are, anionic monomers such as(meth)acrylic acid, 2-(meth)acryloyl ethane sulfonic acid,2-(meth)acryloyl propane sulfonic acid, 2-(meth)acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, and styrene sulfonic acid,and salts thereof; nonionic hydrophilic group-containing monomers suchas (meth)acrylamide, N-substituted (meth)acrylamide, 2-hydroxyethyl(meth)acrylate, and 2-hydroxypropyl(meth)acrylate; and aminogroup-containing unsaturated monomers such asN,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, andN,N-dimethylaminopropyl (meth)acrylamide, and quaternized productsthereof. These monomers may be used either singly or in the form of amixture of two or more members. Among monomers cited above,(meth)acrylic acid, 2-(meth)acryloyl ethane sulfonic acid,2-(meth)acrylamide-2-methylpropane sulfonic acid, and salts thereof,N,N-dimethylaminoethyl(meth)acrylate and quaternizedN,N-dimethylaminoethyl (meth)acrylate, and (meth)acrylamide provepreferable, and acrylic acid and/or the salt thereof are particularlypreferable.

When an acrylic acid salt is used as the monomer, the monovalent salt ofacrylic acid selected among alkali metal salt, ammonium salt, and aminesalt of acrylic acid may be preferably used. More preferably, alkalimetal salts of acrylic acid may be used, and acrylic acid salts selectedamong sodium salt, lithium salt, and potassium salt thereof may beparticularly preferably used.

In the production of a water absorbent resin, a monomer component otherthan the monomers cited above may be used in such an amount as to impaireffects of this invention. As typical examples of such other monomercomponents, hydrophobic monomers such as aromatic ethylenicallyunsaturated monomers having from 8 to 30 carbon atoms, aliphaticethylenically unsaturated monomers having from 2 to 20 carbon atoms,alicyclic ethylenically unsaturated monomers having from 5 to 15 carbonatoms, and alkyl esters of (meth)acrylic acid containing alkyl groupshaving from 4 to 50 carbon atoms may be cited. The proportion of such ahydrophobic monomer is generally in the range of from 0 to 20 parts byweight, based on 100 parts by weight of the ethylenically unsaturatedmonomer. If the proportion of the hydrophobic monomer exceeds 20 partsby weight, water absorbing properties of the produced water absorbentresin would be degraded.

The water absorbent resin which can be used in this invention isinsolubilized by the formation of an internal cross-link. This internalcross-link may be of self-cross-linking type using no cross-linkingagent, or alternatively can be formed by using an internal cross-linkingagent having not less than two polymerizable unsaturated groups and/ornot less than two reactive functional groups in one molecular unit.

The internal cross-linking agent is not particularly restricted. Astypical examples of the inner cross-linking agent,N,N′-methylenebis(meth)acrylamide, N-methylol (meth)acrylamide,glycidyl(meth)acrylate, (poly)ethylene glycol di(meth)acrylate,(poly)propylene glycol di(meth)acrylate, glycerin tri(meth)acrylate,glycerin acrylate methacrylate, polyvalent metal salts of (meth)acrylicacid, trimethylol propane tri(meth)acrylate, triallyl amine, triallylcyanurate, triallyl isocyanurate, triallyl phosphate, ethylene glycoldiglycidyl ether, (poly)glycerol glycidyl ether, and polyethylene glycoldiglycidyl ether may be cited. These internal cross-linking agents maybe used singly or in the form of a mixture of two or more members.

The amount of the internal cross-linking agent to be used is preferablyin the range of from 0.0001 to 1 mol %, more preferably from 0.001 to0.5 mol %, and still more preferably from 0.005 to 0.2 mol %, based onthe total amount of monomer components used in the production of a waterabsorbent resin. If this amount is less than 0.0001 mol %, the internalcross-linking agent may not be introduced satisfactorily into the resin.Conversely, if the amount exceeds 1 mol %, gel strength of the waterabsorbent resin may be too high and the absorption capacity mayconsequently be too low. For the introduction of a cross-linkedstructure into an interior of the polymer by using the internalcross-linking agent, the internal cross-linking agent can be added tothe reaction system prior to, during, or after the polymerization ofmonomers, or after the neutralization of the produced polymer.

For the purpose of producing a water absorbent resin, monomer componentsincluding the monomers and the internal cross-linking agent as mentionedabove are polymerized in an aqueous solution form. Suitablepolymerization initiators include for example water-soluble radicalpolymerization initiators such as persulfates such as potassiumpersulfate, ammonium persulfate, and sodium persulfate; potassiumperacetate, sodium peracetate, potassium percarbonate, sodiumpercarbonate, and t-butyl hydroperoxide; hydrogen peroxide; azocompounds such as 2,2′-azobis(2-amidinopropane)-dihydrochloride, andphotopolymerization initiators such as2-hydroxy-2-methyl-1-phenyl-propan-1-one. The water-soluble radicalpolymerization initiators may be combined with a reducing agent such asa sulfite, L-ascorbic acid, or a ferric salt, to be used as a redox typeinitiator.

The concentration of the monomer in the aqueous monomer solution is notparticularly restricted but is preferably within the range of from 15 to90% by weight, and more preferably from 35 to 80% by weight. If thisconcentration is less than 15% by weight, a lot of heat and time wouldbe required for drying because the resultant hydrogel has an undulylarge content of water.

A method for the polymerization is not particularly restricted but maybe selected among well-known methods such as solution polymerization,reversed-phase suspension polymerization, precipitation polymerization,and bulk polymerization. Among these methods, the aqueous solutionpolymerization which comprises dissolving a monomer in an aqueoussolution and polymerizing it in the aqueous solution, and the reversedphase suspension polymerization may be particularly advantageous due tothe ease of control of polymerization reaction and the performance of aproduced water absorbent resin.

In initiating the polymerization, the polymerization initiator is usedto start the polymerization. Besides the polymerization initiator,active energy rays such as ultraviolet rays, electron radiation, and yrays may be used either singly or in combination with a polymerizationinitiator. Though the temperature in initiating the polymerizationdepends on the kind of polymerization initiator used, it is preferablyin the range of from 15 to 130° C., and more preferably from 20 to 120°C. If the temperature in initiating the polymerization deviates from therange mentioned above, this may result increased amounts of residualmonomer in the produced water absorbent resin. Also, self cross-linkingmay proceed excessively, consequently degrading water absorbingproperties of the produced water absorbent resin.

The “reversed phase suspension polymerization” is a method ofpolymerization in which an aqueous monomer solution is suspended in ahydrophobic organic solvent. It is disclosed in U.S. Pat. Nos.4,093,776, 4,367,323, 4,446,261, 4,683,274, and 5,244,735, for example.The “aqueous solution polymerization” is a method for polymerizing anaqueous monomer solution without using a dispersing solvent. It isdisclosed in U.S. Pat. Nos. 4,625,001, 4,873,299, 4,286,082, 4,973,632,4,985,518, 5,124,416, 5,250,640, 5,264,495, 5,145,906, and 5,380,808,and European Patent Nos. 0 811 636, 0 955 086, and 0 922 717, forexample. The monomers and the initiators which are cited by way ofillustration in these methods of polymerization can be applied to thisinvention.

The aqueous solution polymerization may be performed by polymerizingpartially neutralized acrylic acid or by polymerizing an acidgroup-containing monomer such as acrylic acid and the like andsubsequently neutralizing the resultant polymer with such an alkalicompound as sodium hydroxide or sodium carbonate. Accordingly, the waterabsorbent resin to be used in this invention preferably has an acidgroup and a specific neutralization ratio (mol % of the neutralizedacids group in the whole of acid groups). In this case, theneutralization ratio of the produced water absorbent resin (mol % of theneutralized acids group in the whole of acid groups) is in the range offrom 25 to 100 mol %, and preferably from 50 to 90 mol %, morepreferably of from 50 to 75 mol %, and most preferably from 60 to 70 mol%.

Accordingly, the preferable embodiment of this invention is to provide amethod for the production of a modified water absorbent resin, whichcomprises (i) mixing a water absorbent resin, water, and persulfate as aradical polymerization initiator without addition of an ethylenicallyunsaturated monomer and (ii) irradiating the resultant mixture withactive energy rays, wherein the water absorbent resin contains an acidgroup and has a neutralization ratio (mol % of the neutralized acidsgroup in the whole of acid groups) in the range of 50 to 75 mol %. Afterthe completion of the polymerization, a hydrogel-like cross-linkedpolymer is obtained. While this invention permits this hydrogel-likecross-linked polymer in its unaltered form as a water absorbent resin,the polymer is preferably dried so as to give a water content (% byweight) [100−(Solid content) (% by weight)] which will be specificallydescribed herein below.

Incidentally, in this invention, a water absorbent resin composition isobtained by mixing a water absorbent resin, a water-soluble radicalpolymerization initiator and/or a heat-degradable radical polymerizationinitiator (in the present specification, referred collectively to as“radical polymerization initiator”), and water, which will be describedspecifically herein below. Then, the resultant composition is irradiatedwith active energy rays to modify the water absorbent resin. Thismodification results from the action of active radicals generated fromthe polymerization initiator on the main chain of the polymer. Thismodification, therefore, does not need to be limited to water absorbentresin which is obtained by polymerizing a water-soluble ethylenicallyunsaturated monomer as described above but may be effected on such awater absorbent resin as cross-linked polyvinyl alcohol, cross-linkedpolyethylene oxide, cross-linked polyaspartic acid, and cross-linkedcarboxymethyl cellulose, for example.

The water absorbent resin which can be used in this invention ispreferably a powdery water absorbent resin which is obtained bypolymerizing a monomer having acrylic acid (salt) particularly as itsmain component. The hydrogel-like polymer which is obtained bypolymerization is preferably dried and subsequently pulverized to awater absorbent resin. The drying may be effected by using a drier suchas a hot air drier at a temperature in the range of from 100 to 220° C.,and more preferably from 120 to 200° C.

For pulverization, among shear primary crushers, impact shredders, andhigh speed rotary grinders included in the names of the powderingmachines classified in Table 1.10 of Particle Technology Handbook (firstedition, compiled by Particle Technology Association), the powderingmachines having at least one of the powdering mechanisms such ascutting, shearing, striking, and rubbing can be adopted particularlyfavorably. Among the powdering machines, the powdering machines whichhave cutting and shearing as main mechanisms can be used particularlyadvantageously. A roll mill (roll rotary type) powdering machine may becited as a preferred example.

The water absorbent resin which can be used in this invention ispreferably in a powdery form. More preferably, it is a powdery waterabsorbent resin which contains particles of a diameter in the range offrom 150 to 850 μm (as defined by sieve classification) in a proportionin the range of from 90 to 100% by weight, and particularly preferablyfrom 95 to 100% by weight. When the modified water absorbent resinhaving a particle diameter exceeding 850 μm is used in disposablediapers, for example, it may rupture the top sheet of a diaper. If theparticles of a diameter smaller than 150 μm in a proportion exceeding10% by weight based on weight of the water absorbent resin, the fineparticles may scatter and clog the texture while in use and woulddegrade water absorbing properties of the modified water absorbentresin. The weight average particle diameter of the water absorbent resinmay be in the range of from 10 to 1,000 μm, and preferably from 200 to600 μm. If the weight average particle diameter is less than 10 μm, thismay possibly result in drawbacks regarding safety and health.Conversely, if it exceeds 1,000 μm, the water absorbent resin may not bewell-suited for use in disposable diapers, for example. In thisconnection, as a value of the particle diameter, a value measured by ameasuring method of a particle size distribution specified in theworking example cited below is adopted.

In addition or alternatively, the water absorbent resin to be used inthis invention is preferably obtained by producing a water absorbentresin precursor having a low neutralization ratio, and mixing the waterabsorbent resin precursor with a base. A multifunctionalsurface-treatment agent has been conventionally used for thesurface-treatment (surface cross-linking). The multifunctionalsurface-treatment agent serves to react with a carboxyl group (—COOH) ina water absorbent resin but do not react with the salt thereof (forexample, —COONa). Accordingly, uniform cross-linking can be attained bypreparing an ethylenically unsaturated monomer mixture (for example, amixture of acrylic acid with sodium acrylate) in which —COOH/—COONaratio has been adjusted within a suitable range in advance, polymerizingthe resultant mixture to produce a water absorbent resin having the—COOH and —COONa groups uniformly distributed therein, and subjectingthe resultant water absorbent resin to surface cross-linking with amultifunctional surface-treatment agent. On the other hand, when a waterabsorbent resin is obtained by polymerizing a monomer mixture includingan acid type ethylenically unsaturated monomer like acrylic acid as amain component, and then neutralizing the resultant polymer with analkali compound such as sodium hydroxide and sodium carbonate, theresultant water absorbent resin has a small elutable portion and highgel strength. However, when subjected to surface cross-linking with amultifunctional surface-treatment agent, the absorbent resin likely hasdegraded water absorbency, because the —COOH and —COONa groups are notuniformly distributed in the water absorbent resin. Accordingly, thewater absorbent resin to be produced by the latter method is notdesirably subjected to such a conventional surface cross-linking with amultifunctional surface-treatment agent. Conversely, according to themethod of this invention, since a water-soluble radical polymerizationinitiator or a heat-degradable radical polymerization initiator inducescross-linking by extracting a hydrogen in a main chain to form a radicaland using the radical for coupling, but not by reacting with —COOH, thecross-linking reaction is not affected by whether or not —COOH groupsare uniformly distributed in the water absorbent resin. As a result,according to the method of this invention, a water absorbent resin isobtained by polymerizing a monomer or a monomer mixture including as amain component an acid type ethylenically unsaturated monomer likeacrylic acid to obtain a water absorbent resin precursor having a lowneutralization ratio. This water absorbent resin precursor is thenneutralized with an alkali compound such as sodium hydroxide and sodiumcarbonate, and the resultant modified water absorbent yields high gelstrength and good water absorbency.

If a water absorbent resin precursor having a low neutralization ratiois obtained by polymerizing a monomer or a monomer mixture including asa main component an acid type ethylenically unsaturated monomer, andthen adding a base to the water absorbent resin precursor, the base maybe added either in a solid form or in a liquid form. Preferably, thebase is added in a liquid form, particularly in an aqueous solutionform. When the base is added in an aqueous solution form, adding a baseto a water absorbent resin precursor and producing a water absorbentresin composition can done simultaneously. The base which can be used inthis embodiment is not particularly limited so long as it permits theneutralization of the water absorbent resin precursor having a lowneutralization ratio to a desired neutralization ratio. Well-knowninorganic and organic salt and acid can be used, such as sodiumhydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide,sodium carbonate, potassium carbonate, ammonium carbonate, sodiumbicarbonate, potassium bicarbonate, ammonium bicarbonate, sodiumphosphate, potassium phosphate, ammonium phosphate, sodium borate,potassium borate, ammonium pentaborate, sodium acetate, potassiumacetate, ammonium acetate, sodium lactate, potassium lactate, ammoniumlactate, sodium propionate, potassium propionate, ammonium propionate.These bases can be used singly or in mixed form of two or more members.If a water absorbent resin precursor having a low neutralization ratiois obtained by polymerizing, suitable monomers are an acid typeethylenically unsaturated monomer such as acrylic acid, hydroxide ofmonovalent cation such as sodium hydroxide, potassium hydroxide, lithiumhydroxide, and ammonium hydroxide; and carbonate of monovalent cationsuch as sodium carbonate, potassium carbonate, ammonium carbonate,sodium bicarbonate, potassium bicarbonate, and ammonium bicarbonate ormixtures thereof. Suitable monomers should have good physicalproperties, and permit efficient adjustment of neutralization ratio to adesired level. In this embodiment, the amount of base added is notparticularly limited and can be suitably selected so that the waterabsorbent resin used in the mixing step (i) has a desired neutralizationratio adjusted within the preferable range as mentioned above.

In this invention, the expression “water absorbent resin precursorhaving a low neutralization ratio” is referred to as a water absorbentresin precursor having a low neutralization ratio (mol % of theneutralized acids group in the whole of acid groups) or having noneutralized acid groups (i.e., the neutralization ratio is zero), andtypically referred to as a water absorbent resin precursor having aneutralization ratio (mol % of the neutralized acids group in the wholeof acid groups) of from 0 to 50 mol %, more preferably from 0 to 20 mol%. Such a water absorbent resin precursor having a low neutralizationratio can be obtained by the same method as mentioned above by using amonomer mixture including as a main component an acid group-containingmonomer like acrylic acid wherein a neutralization ratio is preferablyadjusted within the above range.

The water content of the water absorbent resin prior to the modificationto be used in the method for production of a modified water absorbentresin contemplated by this invention has no particular restriction solong as the water absorbent resin has fluidity. The water absorbentresin after being dried at 180° C. for three hours has a water contentfalling in the preferable range of from 0 to 50% by weight, from 0 to40% by weight, from 0 to 30% by weight, from 0 to 20% by weight, from 0to 10% by weight, and more preferably from 0 to 5% by weight in thisorder.

The water absorbent resin to be used in this invention is not limited tothe product of the method as described above but may be any productobtained by some other method. While the water absorbent resin which isobtained by the method described above is a water absorbent resin havingundergone no surface cross-linking, for use in the method for producinga modified water absorbent resin of this invention, the water absorbentresin which has undergone surface cross-linking in advance with apolyhydric alcohol, a polyvalent epoxy compound, an alkylene carbonate,or an oxazolidone compound can be adopted.

(b) Water Absorbent Resin Composition

In a method for the production of a modified water absorbent resinaccording to the present invention, in the step (i), a water absorbentresin composition is obtained by mixing water and a radicalpolymerization initiator (a water-soluble radical polymerizationinitiator and/or a heat-degradable radical polymerization initiator)with the water absorbent resin, without addition of an ethylenicallyunsaturated monomer.

Hitherto, the surface cross-linking of a water absorbent resin has beengenerally effected by using a surface cross-linking agent. Theincorporation of the surface cross-linking agent results in strong,chemical binding between the functional groups present on the surface ofresin and the surface cross-linking agent, thereby introducing a stablesurface cross-link structure into the resin surface. Then, by properlyselecting a chain length of the surface cross-linking agent, thedistance between cross-links can be adjusted easily. By adjusting anamount of the surface cross-linking agent to be incorporated, thecross-link density can be controlled. This invention, however, permitsthe modification of a water absorbent resin, specifically theintroduction of a cross-link structure to the surface of the waterabsorbent resin, by merely using a radical polymerization initiatorwithout requiring the incorporation of the surface cross-linking agent.Further, by additionally adding water to obtain a water absorbentcomposition and irradiating the water absorbent resin composition withactive energy rays, a cross-linked structure can be effectivelyintroduced to the surface of the water absorbent resin particles and atthe same time, the produced modified water absorbent resin has improvedwater absorption properties. Moreover, the addition of water in arelatively large amount to the water absorbent resin in the step (i)permits the efficient introduction of a cross-linking structure on asurface of the water absorbent resin in the step (ii) described indetail below, and thus also results in shortened irradiation timerequired for improving absorbency against pressure (AAP) and the salineflow conductivity (SFC) of the modified water absorbent resin to adesired level.

This invention uses the expression “without addition of an ethylenicallyunsaturated monomer” with the object of preventing a radicalpolymerization initiator from reacting with an ethylenically unsaturatedmonomer to avoid the consumption of the radical polymerization initiatorthat is activated by the irradiation with active energy rays prior tothe action on the surface of the water absorbent resin in the step (ii).

In the step (i), water is mixed with a water absorbent resin. In thiscase, mixing of a water absorbent resin and water may be conducted byadding water alone, or by adding water in a form of an aqueous solutioncontaining another component. As the aqueous solution, for example, anaqueous solution containing a radical polymerization initiator, anaqueous solution containing a mixing aid, and the like may be included.

In the step (i), the amount of water mixed with a water absorbent resinpreferably exceeds 20_parts by weight and is not more than 100 parts byweight, based on 100 parts by weight of the water absorbent resin (asreduced to as 100 parts by weight of a solid content). The addition ofwater in a relatively large amount to the water absorbent resin in thestep (i) permits the efficient introduction of a cross-linking structureon a surface of the water absorbent resin in the step (ii) described indetail below, and thus a shortened irradiation time. The preferableamount of water mixed with the water absorbent resin exceeds 20 parts byweight and is not more than 70 parts by weight, or exceeds 20 parts byweight and is not more than 50 parts by weight, or exceeds 20 parts byweight and is not more than 40 parts by weight, or exceeds 20 parts byweight and is not more than 30 parts by weight, in this order, based on100 parts by weight of a water absorbent resin (referred to 100 parts byweight of a solid content). If the amount of water exceeds 100 parts byweight, a large amount of energy may be necessary during a drying stepafter irradiation with active energy rays. In addition, the waterabsorbent resin may possibly be decomposed.

Further, in step (i), in addition to the water, a water-soluble radicalpolymerization initiator and/or a heat-degradable radical polymerizationinitiator are mixed as a radical polymerization initiator with the waterabsorbent resin composition. Incidentally, hereinafter, “a water-solubleradical polymerization initiator and/or a heat-degradable radicalpolymerization initiator” are sometimes called collectively as radicalpolymerization initiator.

In this step, when “a water-soluble radical polymerization initiator” ismixed with a water absorbent resin, the initiator can be easilydispersed uniformly on the surface of the water absorbent resin whichexcels in hydrophilic property and water absorbing property. Thus, awater absorbent resin excelling in water absorbing properties can beproduced.

The water-soluble radical polymerization initiator to be used in thisinvention has solubility in water (25° C.) of not less than 1% byweight, preferably not less than 5% by weight, and more preferably notless than 10% by weight. Typical examples are, persulfates such asammonium persulfate, sodium persulfate, and potassium persulfate;hydrogen peroxide; and water-soluble azo compounds such as2,2′-azobis-2-amidinopropane dihydrochloride and2,2′-azobis[2-(2-imidazolin-2-yl) propane]dihydrochloride. The use of apersulfate is particularly preferable as the modified water absorbentresin has good water absorption properties including absorbency ofphysiological saline against pressure (in this specification, referredsimply to as “absorbency against pressure”), and saline flowconductivity.

The term heat-degradable radical polymerization initiator to be used inthis invention is a compound which generates a radical by heating. Aheat-degradable radical polymerization initiator having 10 hourhalf-life decomposition temperature in the range of 0 to 120° C., morepreferably 20 to 100° C., may be preferably used in this invention. Inconsideration of temperature during the irradiation with active energyrays, a heat-degradable radical polymerization initiator having 10 hourhalf-life decomposition temperature in the range of 40 to 80° C. can beparticularly preferably used in this invention. If the lower limit of 10hour half-life decomposition temperature is less than 0° C. (lowerlimit), the heat-degradable radical polymerization initiator would betoo unstable during storage. Conversely, if the upper limit thereofexceeds 120° C. (upper limit), the chemical stability of theheat-degradable radical polymerization initiator may be too high.

In the step, when “a heat-degradable radical polymerization initiator”is mixed with a water absorbent resin, the surface modification can becarried out at a low temperature for a short period of time, and theresultant modified water absorbent resin can manifest high gel strengthand good water-absorbing properties. The heat-degradable radicalpolymerization initiator to be used in this invention may be eitheroil-soluble or water-soluble. The decomposition rate of an oil-solubleheat-degradable radical polymerization initiator is less sensitive to apH value and ion strength as compared to that of a water-solubleheat-degradable radical polymerization initiator. However, awater-soluble heat-degradable radical polymerization initiator may bemore preferably used in respect of its permeability to a water absorbentresin because the water absorbent resin is hydrophilic.

The heat-degradable radical polymerization initiator is relativelyinexpensive and the process and devices for the production thereof canbe simplified because the strict light-shielding is not always required,as compared to a compound which has been commercially available as aphoto-degradable radical polymerization initiator. Representativeexamples of the heat-degradable radical polymerization initiator arepersulfates such as sodium persulfate, ammonium persulfate, andpotassium persulfate; percarbonates such as sodium percarbonate;peracetates such as peracetic acid, and sodium peracetate; hydrogenperoxide; and azo compounds such as 2,2′-azobis(2-amidinopropane)dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and 2,2′-azobis(2-methylpropionitrile. Amongthe heat-degradable radical polymerization initiators cited above,persulfates including sodium persulfate, ammonium persulfate, andpotassium persulfate, and azo compounds including2,2′-azobis(2-amidinopropane) dihydrochloride,2,2′-azobis[2-(2-imidazolin-2-yl) propane]dihydrochloride, and2,2′-azobis(2-methylpropionitrile) which have 10 hour half-lifedecomposition temperature in the range of 40 to 80° C. can be usedpreferably. Particularly, persulfates may be preferably used in respectof excellent absorbency of physiological saline against pressure, salineflow conductivity, and free swelling capacity.

The amount of the radical polymerization initiator is preferably in therange of from 0.01 to 20 parts by weight, more preferably from 0.1 to 15parts by weight, and particularly preferably from 1 to 10 parts byweight, based on 100 parts by weight of the water absorbent resin. Ifthe amount of the radical polymerization initiator to be mixed is lessthan 0.01 parts by weight, the water absorbent resin may not besufficiently modified even upon exposure to the active energy rays.Conversely, if the amount of the radical polymerization initiator to bemixed exceeds 20 parts by weight, water absorbing properties of themodified water absorbent resin may possibly be degraded.

In this invention, by essentially using the water-soluble radicalpolymerization initiator and/or a heat-degradable radical polymerizationinitiator, excellent properties can be accomplished compared to caseswherein such radical polymerization initiators are omitted, for example,the case of using solely an oil-soluble photopolymerization initiator.Incidentally, the term “oil-soluble photopolymerization initiator” asused herein means a compound having water-solubility of less than 1% byweight.

While this invention essentially uses a water-soluble radicalpolymerization initiator and/or a heat-degradable radical polymerizationinitiator, an initiator other than the radical polymerization initiatorcan be additionally used. As typical examples of the otherpolymerization initiators which can be additionally used, arephotopolymerization initiators such as oil-soluble benzoin derivatives,benzyl derivatives, and acetophenone derivatives, and oil-solubleorganic peroxides such as oil-soluble ketone peroxide, peroxyketal,hydroperoxide, dialkyl peroxide, peroxy esters, and peroxycarbonate.These photopolymerization initiators may be commercially availableproducts such as, for example, products from Ciba Specialty Chemicalssold under the trademark designations of Irgacure 184(hydroxycyclohexyl-phenyl ketone) and Irgacure 2959(1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-on).

When an additional initiator is to be used in combination in thisinvention, the amount of the other initiator to be used is in the rangeof from 0 to 20 parts by weight, preferably from 0 to 15 parts byweight, and particularly preferably from 0 to 10 parts by weight, basedon 100 parts by weight of the water absorbent resin. This ratecorresponds to a smaller amount than the radical polymerizationinitiator such as, for example, not more than ½, further not more than1/10, and particularly not more than 1/50 of the weight ratio of thewater-soluble radical polymerization initiator. When a water-solubleradical polymerization initiator and/or a heat-degradable radicalpolymerization initiator are to be used in combination, the amount ofthe radical polymerization initiator is referred to a total amountthereof.

While the mixing of the radical polymerization initiator and the waterabsorbent resin mentioned above may be accomplished by mixing theradical polymerization initiator with the unmodified water absorbentresin, it is preferably performed by dissolving the initiator in anaqueous solution and then mixing the resultant aqueous solution with thewater absorbent resin. Since the water absorbent resin is capable ofabsorbing water, the radical polymerization initiator can be uniformlydispersed on the surface of the water absorbent resin and uniformlymixed with the water absorbent resin by mixing the radicalpolymerization initiator in an aqueous solution form. The aqueoussolution may contain, besides water, some other solvent in an amountwhich does not impair solubility of the radical polymerizationinitiator.

Further, when a radical polymerization initiator is added in a form ofan aqueous solution, the amount of water in an aqueous solution used isnot limited. In this connection, a form of mixing water into a waterabsorbent resin is not limited to a case where mixing is conducted in aform of an aqueous solution containing a radical polymerizationinitiator. After mixing a radical polymerization initiator and a waterabsorbent resin, water or an aqueous solution may be mixed therewith.Therefore, a hydrogel-like cross-linked product is obtained bypolymerizing a monomer component, drying to give a water content of 0 to50% by weight, and then directly mixing with a radical polymerizationinitiator, to obtain a water absorbent resin composition.

For improving the mixing property of the aqueous solution with a waterabsorbent resin composition, a mixing aid may be added to the waterabsorbent resin composition. Although the time of adding a mixing aid isnot particularly critical, the mixing aid is preferably added at thesame time as or prior to the mixing step (i).

The mixing aid is not particularly limited, as long as it is awater-soluble or water-dispersible compound except an ethylenicallyunsaturated monomer or a radical polymerization initiator, and it canrepress the agglomeration of the water absorbent resin with water andimprove the mixing of the aqueous solution with the water absorbentresin. The mixing aid is preferably a water-soluble or water-dispersiblecompound. As such a water-soluble or water-dispersible compound,surfactants, water-soluble polymers, hydrophilic organic solvents,water-soluble inorganic compounds, inorganic acids, inorganic acidsalts, organic acids, and organic acid salts can be typically used. Inthis specification, the term “water-soluble compound” is referred to asa compound having solubility in 100 g of water at room temperature ofnot less than 1 g, preferably not less than 10 g. Since the addition ofthe mixing aid can repress the agglomeration of the water absorbentresin with water and induce the uniform mixing of the aqueous solutionwith the water absorbent resin, the active energy rays, in thesubsequent step, can be radiated equally and evenly to the waterabsorbent resin and thus the uniform surface cross-linking of the entirewater absorbent resin can be attained.

When a mixing aid is to be used, the form of the mixing aid is notparticularly limited, and it may be used in a powdery form, or may bedissolved, dispersed, or suspended in a solution. Preferably, it is usedin the form of an aqueous solution.

Further, in the case of using a mixing aid, the order of the addition ofthe mixing aid is also not particularly limited. Any method such as amethod which comprises adding a mixing aid to a water absorbent resin,and then adding and mixing water and a radical polymerization initiator(in some cases, an aqueous solution containing them) to the mixture, anda method which comprises dissolving a mixing aid in an aqueous solution,and simultaneously mixing the resultant solution with a water absorbentresin can be used.

As the surfactant to be used herein, at least one kind of surfactantwhich is selected from the group consisting of nonionic surfactants andanionic surfactants having an HLB of not less than 7 may be adopted.Typical examples of such surfactants are sorbitan aliphatic esters,polyoxyethylene sorbitan aliphatic esters, polyglycerin aliphaticesters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenolethers, polyoxyethylene acyl esters, sucrose aliphatic esters, higheralcohol sulfuric esters, alkyl naphthalene sulfonates,alkylpolyoxyethylene sulfate, and dialkyl sulfosuccinates. Among thesesurfactants, polyoxyethylene alkyl ethers can be preferably used. Thenumber average molecular weight of the polyoxyethylene alkyl ether ispreferably in the range of 200 to 100,000, more preferably 500 to10,000. If the number average molecular weight is too large, thesolubility in water may decrease and thus the mixing with the waterabsorbent resin may become inefficient because the concentration of thesurfactant in the solution can not be increased. Also, the viscosity ofthe solution may be increased. Conversely, if the number averagemolecular weight is too small, the surfactant may become less effectiveas a mixing aid.

Typical examples of the water-soluble polymer are polyvinyl alcohol,polyethylne oxide, polyethylene glycol, polypropylene glycol,polyacrylamide, polyacrylic acid, sodium polyacrylate, polyethyleneimine, methyl cellulose, carboxymethyl cellulose, hydroxyethylcellulose, hydroxypropyl cellulose, dextrin, sodium alginate, andstarch. Among these polymers, polyethylene glycol can be preferablyused. The number average molecular weight of the polyethylene glycol,like polyoxyethylene alkyl ether, is preferably in the range of 200 to100,000, more preferably 500 to 10,000.

Typical examples of the hydrophilic organic solvent are alcohols such asmethyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butylalcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetoneand methylethyl ketone; ethers such as dioxane, alkoxy(poly)ethyleneglycol, and tetrahydrofuran; amides such as ε-caprolactam andN,N-dimethyl formamide; sulfxides such as dimethyl sulfoxide; andpolyhydric alcohols such as ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentane diol, glycerin,2-butene-1,4-diol, 1,3-butane diol, 1,4-butane diol, 1,5-pentane diol,1,6-hexane diol, 1,2-cyclohexane dimethanol, 1,2-cyclohexanol,trimethylol propane, diethanol amine, triethanol amine,polyoxypropylene, pentaerythritol, and sorbitol. These hydrophilicorganic solvents may be used either singly or in the form of a mixtureof two or more members.

Typical examples of the water-soluble inorganic compound are alkalimetal salts such as sodium chloride, sodium hydrogen sulfate, and sodiumsulfate, ammonium salts such as ammonium chloride, ammonium hydrogensulfate, and ammonium sulfate, alkali metal hydroxides such as sodiumhydroxide and potassium hydroxide, polyvalent metal salts such asaluminium chloride, polyaluminium chloride, aluminium sulfate, potassiumalum, calcium chloride, alkoxy titanium, zirconium ammonium carbonate,zirconium acetate, and non-reducible alkali metal salt pH buffer agentssuch as hydrogencarbonate, dihydrogen phosphate, and monohydrogenphosphate.

Further, typical examples of the inorganic acid (salt) are hydrochloricacid, sulfuric acid, phosphoric acid, carbonic acid, and boric acid, andthe salts thereof, for example, alkali metal salts thereof, and alkaliearth metal salts thereof may be cited. As typical examples of theorganic acid (salt), acetic acid, propionic acid, lactic acid, citricacid, succinic acid, malic acid, and tartaric acid, and the saltsthereof, for example, alkali metal salts thereof, and alkali earth metalsalts thereof.

Among the compounds cited above, at least one water-soluble orwater-dispersible compound selected from the group consisting ofpolyoxyethylene alkyl ethers, polyethylene glycol, water-solublepolyvalent metals, sodium chloride, ammonium hydrogen sulfate, ammoniumsulfate, sulfuric acid, and hydrochloric acid may be preferably used asthe mixing aid.

These mixing aids can be used singly or in the mixed form of two or moremembers. The amount of the mixing aid to be added is not particularlylimited as long as it can repress the aggregation of the water absorbentresin with water, and improves the mixing of the aqueous solution withthe water absorbent resin, as mentioned above. Typically, the mixing aidis preferably added in an amount in the range of from 0.01 to 40 partsby weight, more preferably from 0.1 to 5 parts by weight, to 100 partsby weight of the water absorbent resin.

In the step (i) according to this invention, the conditions for mixing awater absorbent resin, water, and a radical polymerization initiator,and optionally a mixing aid are not critical. For example, the mixingtemperature in the step (i) is preferably in the range of from 0 to 150°C., or from 10 to 120° C., or from 20 to 100° C., or from 30 to 90° C.,or from 40 to 70° C., in this order. If the mixing temperature exceeds150° C., the water absorbent resin may be degraded by heat, and thesurface water content of the water absorbent resin in the step (ii) maybe too low due to evaporation of water and the like. Conversely, if themixing temperature is less than 0° C., water would be condensed, therebyinhibiting the stable operation. Carrying out the mixing step at anelevated temperature is preferred because a radical polymerizationinitiator can act also with small radiation amounts due to the heat.Accordingly, in such a case, a mixing/irradiation system may bepreferably closed so as to repress excessive leakage of steam and toincrease a surface water content of a water absorbent resin in the step(ii) to a level of not less than 3.0% by weight. The temperatures ofwater absorbent resin and water prior to the step (i) are not alsoparticularly limited. For example, the temperature of water absorbentresin prior to the step (i) is preferably in the range of from 0 to 150°C., or from 10 to 120° C., or from 20 to 100° C., or from 50 to 100° C.,in this order. If the temperature of water absorbent resin prior to thestep (i) exceeds 150° C., the water absorbent resin may be degraded byheat. Conversely, if the mixing temperature is less than 0° C., watermay be condensed, thereby inhibiting a stable operation. The temperatureof water prior to the step (i) is preferably in the range of from 5 to80° C., more preferably from 10 to 60° C., particularly preferably from20 to 50° C. If the temperature of water prior to the step (i) exceeds80° C., excessive amounts of water may evaporate prior to the mixingstep (i) and thus a sufficient amount of water can not be mixed with awater absorbent resin resulting in a surface water content of the waterabsorbent resin which is too low. Conversely, if it is less than 5° C.,water may be condensed, thereby inhibiting stable operation. Further,the mixing time in the step (i) is not also particularly limited as longas the above components can be mixed uniformly. Typically, the mixingtime is preferably in the range of from 0.1 second to 60 minutes, morepreferably from 1 second to 30 minutes, further more preferably from 2seconds to 20 minutes, most preferably from 5 seconds to 10 minutes. Ifthe mixing time is less than the lower limit, water absorbent resin,water, and a radical polymerization initiator, and optionally a mixingaid may not be mixed uniformly. Conversely, if the mixing time exceedsthe upper limit and becomes unduly long, an excess amount of water wouldpenetrate into an inner part of the water absorbent resin, therebyunduly decreasing the water content on the surface.

Suitable devices for mixing a water absorbent resin, water, and aradical polymerization initiator are, for example, V-shape mixer, ribbontype mixer, screw type mixer, rotary circular plate type mixer,air-current type mixer, batch kneader, continuous kneader, paddle typemixer, or space type mixer.

(c) Active Energy Rays

It is known that in production of a water absorbent resin, the rate ofpolymerization can be increased upon exposure to active energy rays. Forexample, by adding a polymerizable monomer component, an internalcross-linking agent and a photopolymerization initiator and irradiatingthe resultant mixture with active energy rays such as ultraviolet rays,electron radiation, or y rays, a water-insoluble absorbent resin havinginternal cross-links can be prepared. Then, as a method forcross-linking the surface of the water absorbent resin, the formation ofsurface cross-links can be achieved by using a surface cross-linkingagent and promoting the relevant reaction by application of heat. Forthe surface cross-linking of the water absorbent resin, compounds suchas polyhydric alcohols, polyvalent glycidyl ethers, haloepoxy compounds,and polyvalent aldehydes which contain a plurality of functional groupsin one molecular unit may be used. Generally, by heating at from 100 to300° C., these functional groups can react with carboxyl groups presenton the surface of the water absorbent resin to give rise to across-linked structure on the surface of the water absorbent resin. Inthis invention, however, a cross-linked structure can be formed on asurface of a water absorbent resin by combining a radical polymerizationinitiator and exposure with active energy rays without requiring thepresence of a surface cross-linking agent and a polymerizable monomer.By the method of this invention, absorbency against pressure (AAP) andthe saline flow conductivity (SFC) of the modified water absorbent resincan be improved.

According to the method of this invention, a surface water content of awater absorbent resin in the water absorbent resin composition iscontrolled to be above a predetermined value when irradiated with activeenergy rays.

Specifically, in the irradiating step, a surface water content of thewater absorbent resin in the water absorbent resin composition iscontrolled to be not lower than 3.0% by weight. The surface watercontent may be controlled to a level of not lower than 3.0% by weight atany point of time in the irradiating step, and it is not necessary tocontrol to a level of not lower than 3.0% by weight throughout thecourse from the beginning to the end of the irradiating step. When asurface water content of the water absorbent resin is controlled to alevel of not lower than 3.0% by weight throughout the course from thebeginning to the end of the irradiating step, the modification (forexample, introduction of a cross-linking structure) of a surface of thewater absorbent resin may not be carried out efficiently conducted.

As described above, the surface water content may be controlled to alevel of not lower than 3.0% by weight at least at any point of time inthe irradiating step. It is preferably controlled to a level of notlower than 3.5% by weight, and more preferably not lower than 4.0% byweight. The upper limit of the surface water content is not particularlycritical, and it may be appropriately selected in accordance withpurposes. However, the surface water content is typically not higherthan 60.0% by weight, preferably not higher than 50.0% by weight, morepreferably not higher than 40.0% by weight, and further preferably nothigher than 30.0% by weight. When the surface water content is too high,water absorbent resin particles may adhere or agglomerate, andirradiation with active energy rays may not be effectively carried out.

In a typical embodiment of the present invention, the surface watercontent at the beginning of the irradiating step is controlled so as tobe within the above-described range. It should be noted that during theirradiating step, the surface water content may be varied. Namely, thesurface water content may be increased or decreased as compared to theinitial water content. However, the surface water content should bewithin the above-described range. The surface water content iscontrolled to be within the above-described range, in preferably notlower than 30%, more preferably not lower than 60%, further preferablynot lower than 90%, and particularly preferably 100% (that is, the wholeperiod), of the whole period of the irradiating step.

In this invention, the term “surface water content” is referred to aweight percentage of a water amount existing in the vicinity of asurface of a particle based on a weight of a water absorbent resinparticle. It is essentially different from a concept of a water amountor the water content in the whole particle. The surface water content ismeasured by the method specified in the working example cited below. Themeasuring method is briefly explained, as follows. Water is extracted byadding a hydrophilic organic solvent to the water absorbent resincomposition obtained in the step (i), a water amount in the extract isquantitatively determined by Karl Fischer method, and thus a value ofthe surface water content can be calculated.

In the irradiating step, the method of controlling a surface watercontent of a water absorbent resin is not especially limited. Forexample, to attain a preferable surface water content a sufficientamount of water can be added into a water absorbent resin compositionobtained in the step (i), or permeation of water into an inner part of awater absorbent resin particle can be promoted; or water evaporationinto the atmosphere can be suppressed in the mixing step (i) and in theirradiating step (ii). Since an extent of permeation of water into aninner part of a water absorbent resin particle is influenced by time andtemperature, it is preferable to control the temperature in a systemduring the mixing step (i). Further, to suppress evaporation of water,it is preferable have a closed system, to control the mixing time andthe temperature in the system. In the irradiating step (ii), when astirring apparatus having a box-like or a cylinder-like shape, forinstance, a closed system can be obtained by covering an opening partfor irradiation with a material capable of transmitting active energyrays, such as quartz glass. Promoting permeation of water to an innerpart of a water absorbent resin can be achieved by extending the mixingstep (i); or by putting the water absorbent resin composition in aclosed system: or by heat treating the water absorbent resin compositionat a temperature of not higher than a boiling point of water. On theother hand, promoting diffusion of water from a surface can be achievedby subjecting an air stream to a water absorbent resin composition; orby putting the water absorbent resin composition in an open system: orby heat treating the water absorbent resin composition at a temperatureof not lower than a boiling point of water.

To monitor the a surface water content, the water absorbent resincomposition may be dried to a certain range, or a predetermined amountof water may be added to a water absorbent resin composition, dependingon the monitored value. Moreover, when water is added, penetration anddiffusion of water from a surface to an inner part of the waterabsorbent resin may appropriately be controlled, or evaporation of waterfrom a surface of a water absorbent resin may appropriately becontrolled.

In this invention, the irradiation with active energy rays may beconducted while water absorbent resin, water and radical polymerizationinitiator are mixed, or the irradiation may be conducted after mixing atleast two of these.

As typical examples of active energy rays, ultraviolet rays, electronradiation, and y rays may be cited. These active energy rays may be usedeither singly or in the form of a combination of two or more members.Among these active energy rays, ultraviolet rays and electron radiationprove advantageous. In consideration of influence of active energy rayson human body, ultraviolet rays are more preferable and ultraviolet rayshaving a wavelength not exceeding 300 nm and particularly preferably inthe range of 180-290 nm are particularly preferable.

As regards irradiating conditions, when the ultraviolet rays are used,intensity of irradiation is preferably in the range of 3-1,000 mW/cm²,and dose of irradiation is preferably in the range of 100-10,000 mJ/cm².Typical examples of the device for irradiation with ultraviolet rays arehigh-pressure mercury-vapor lamp, low-pressure mercury-vapor lamp, metalhalide lamps, xenon lamp, and halogen lamps. As long as ultravioletrays, preferably ultraviolet rays of a wavelength of not more than 300nm, are used, the use of additional different radiation types ordifferent wavelengths is not particularly restricted. If electronradiation is used, voltage of acceleration is preferably in the range of50-800 kV and absorbed dose is preferably in the range of 0.1-100 Mrad.

Generally, the duration of irradiating with active energy rays ispreferably not less than 0.1 minute and less than 60 minutes, morepreferably not less than 0.2 minute and less than 30 minutes, and morepreferably not less than 1 minute and less than 15 minutes. Contrarythereto, conventional surface cross-linking using a conventional surfacecross-linking agent, typically requires more time. For surfacecross-linking by irradiation with active energy rays, no application ofheat is required. The irradiation of active energy rays, however,possibly results in generation of radiant heat. Generally, a waterabsorbent resin can be treated at a temperature preferably less than150° C., more preferably less than 120° C., still more preferably in therange of room temperature to 100° C., and particularly preferably in therange of 50-100° C. Thus, this invention allows a treating temperaturebelow the typical temperature of conventional surface cross-linking.

Throughout irradiation with active energy rays, the water absorbentresin is preferably stirred or otherwise agitated to ensure uniformirradiation with the active energy rays. Typical examples of stirringdevices are shaking mixer, shaking feeder, ribbon type mixer, conicalribbon type mixer, screw type mixing extruder, air current type mixer,batch kneader, continuous kneader, paddle type mixer, high-speedfluidifying mixer, and buoyant fluidifying mixer.

Further, irradiation may originate from the surrounding of an apparatus,while the water absorbent resin composition is in an apparatus having aform of a box or a cylinder. In this case, to make the mixture flow, apressure of gas such as air or the like may be utilized, as is used inflowing a powder with air. When air is used, it is preferable tohumidify the air to prevent the water absorbent resin composition fromdrying. When irradiation with active energy rays is conducted from manydirections, subsequently or, preferably simultaneously, uniform surfacetreatment can be conducted in a short period. In this connection, thematerial of which the above-described apparatus is made is notparticularly critical, as long as it does not obstruct irradiation withactive energy rays onto the water absorbent resin composition. Forexample, quartz glass would be a suitable material.

(d) Other Treatment

After irradiation with active energy rays, the water absorbent resin mayoptionally be subjected to heat treatment at a temperature in the rangeof 50-250° C. for drying.

Further, after the irradiation with active energy rays, a waterabsorbent resin may be further surface cross-linked by using aconventional surface cross-linking agent such as polyhydric alcohols,polyvalent epoxy compounds, and alkylene carbonates.

In the method for producing a modified water absorbent resin of thepresent invention, a water absorbent resin may be added with an agentfor enhancing liquid-permeability before, after or during irradiationwith active energy rays. Typical examples of such an agent are mineralssuch as talc, kaolin, fuller's earth, bentonite, activated clay, barite,natural asphaltum, strontium ore, ilmenite, and pearlite; aluminumcompounds such as aluminum sulfates 14-18 hydrates (or anhydrides),potassium aluminum sulfates 12 hydrate, sodium aluminum sulfate 12hydrate, aluminum chloride, aluminum polychloride, and aluminum oxide,and aqueous solutions thereof; other polyvalent metal salts; hydrophilicamorphous silica (such as, for example, a product by dry method made byTokuyama K.K. and sold under the trademark designation of “ReolosilQS-20” and products by precipitation method made by DEGUSSA Corp. andsold under the trademark designation of “Sipernat 22S” and “Sipernat2200”); and oxide composites such as silicon oxide-aluminumoxide-magnesium oxide composite (such as, for example, a product made byENGELHARD Corp. and sold under the trademark designation of “Attagel#50”), silicon oxide-aluminum oxide composite, and siliconoxide-magnesium oxide composite. The amount of such aliquid-permeability enhancing agent would preferably be in the range offrom 0 to 20 parts by weight, more preferably from 0.01 to 10 parts byweight, and particularly preferably from 0.1 to 5 parts by weight with100 parts by weight of a water absorbent resin which has been modified.The liquid-permeability enhancing agent can be added in the form of anaqueous solution if it is water-soluble or in the form of powder orslurry when it is water-insoluble. The liquid-permeability enhancingagent may also be added in a mixed form with a radical polymerizationinitiator. Other additives such as antibacterial agent, deodorant, andchelating agent may additionally be used in an amount in the range asmentioned above for the liquid-permeability enhancing agent.

(e) Modified Water Absorbent Resin

According to the method for producing a modified water absorbent resinof this invention, the produced water absorbent resin has improvedabsorbency against pressure. It has been hitherto known that theformation of surface cross-linking results in slightly lowering the freeswelling capacity while increasing the ability to retain absorbed liquideven under pressed state, namely absorbency against pressure (AAP). Bythe method of this invention, the absorbency against pressure of 4.83kPa of the water absorbent resin can be improved by not less than 1 g/gcomparing with the absorption against pressure of the resin prior to themodification. It is believed that an increase in AAP indicates thatsurface cross-linking has taken place. The increase in the absorbencyagainst pressure after the modification is preferably not less than 8g/g, more preferably not less than 12 g/g, still more preferably notless than 15 g/g, and particularly preferably not less than 20 g/g, mostpreferably not less than 22 g/g. The modified water absorbent resin ofthis invention may exhibit absorbency against pressure of 4.83 kPa inthe range of 8-40 g/g.

The centrifuge retention capacity (CRC) of the modified water absorbentresin is preferably not more than 50 g/g, more preferably not more than40 g/g, still more preferably not more than 35 g/g. Although the lowerlimit thereof is not particularly limited, it is preferably not lessthan 10 g/g, more preferably not less than 20 g/g, still more preferablynot less than 25 g/g. If the centrifuge retention capacity (CRC) exceeds50 g/g, gel strength might be decreased, decreasing absorbency againstpressure. On the hand, if the centrifuge retention capacity (CRC) isless than 10 g/g, sufficient water absorption capacity may not beobtained.

The modified water absorbent resin which is obtained by this inventionhas a saline flow conductivity (SFC) preferably of not less than 10(×10⁻⁷·cm³·s·g⁻¹), more preferably not less than 30 (×10⁻⁷·cm³·s·g⁻¹),and still more preferably not less than 50 (×10⁻⁷·cm³·s·g⁻¹),particularly preferably not less than 70 (×10⁻⁷·cm³·s·g⁻¹), mostpreferably not less than 100 (×10⁻⁷·cm³·s·g⁻¹). The value is to bedetermined by the method specified in the working example cited hereinbelow.

Further, the modified water absorbent resin which is obtained by thisinvention has low residual monomer content. It is believed that this isdue to a reaction between the remaining monomers in the water absorbentresin with the initiator radicals formed by irradiation on a radicalpolymerization initiator with active energy rays. Since the waterabsorbent resin is used in hygienic materials such as disposable diaper,the residual monomer content is preferably as small as possible in termsof odor and safety. While a residual monomer content of water absorbentresin as a base polymer is generally in the range of 200 to 500 ppm, theresidual monomer content of the water absorbent resin surface-treated bythis invention is typically not more than 200 ppm (the lower limit is 0ppm). The residual monomer content of the modified water absorbent resinis preferably not more than 200 ppm, more preferably not more than 150ppm, particularly not more than 100 ppm (the lower limit is 0 ppm).

Further, the modified water absorbent resin which is obtained by thisinvention has a smaller solid content as compared with a modified waterabsorbent resin obtained by conventional surface cross-linking. This isbecause according to the method of this invention, the reaction does notrequire an elevated temperature and thus the water contained in theaqueous solution which is added to a water absorbent resin does notevaporate or only to a small degree Due to the large water content ofthe water absorbent resin, there is only a small amount of fine powderhaving a particle size of not more than 150 p.m. Such particles are notdesirable in terms of health. Also, the generation of static electricityon particle surface which causes blocking during the pneumatic conveyingcan be prevented, and the degradation of physical properties by physicaldamage during the pneumatic conveying can be repressed. The solidcontent of the modified water absorbent resin is preferably not morethan 95%, more preferably not more than 93%, particularly not more than91%. Although the lower limit is not critical, a solid content of notmore than 70% may lead to decreased absorbency per weight of the waterabsorbent resin.

The properties of the surface-treated water absorbent resin which isobtained by this invention can be further adjusted by treatmentconditions such as by selecting a suitable unmodified water absorbentresin and agglomeration and molding processes of a water absorbent resinafter surface cross-linking. Generally, the modified water absorbentresin is in a powdery form. This powder has a weight average particlediameter (specified by classification with sieves) in the range of from10 to 1,000 μm, and preferably from 200 to 600 μm. In this powder, thecontent of particles having diameters of from 150 to 850 μm ispreferably in the range of from 90 to 100% by weight, and morepreferably from 95 to 100% by weight, based on the weight of the waterabsorbent resin.

The method of this invention effects an agglomerating a fine powdergenerated in the production of a water absorbent resin during the courseof surface cross-linking of the water absorbent resin. Accordingly, evenif the water absorbent resin prior to the modification happens tocontain a fine powder, the method for producing a modified waterabsorbent resin of this invention permits the agglomeration of thecontained fine powder, which can lead to an decreased amount of finepowder in the resultant modified water absorbent resin. Thus, theparticle size distribution of the produced modified water absorbentresin is shifted toward a larger particle size as compared with thewater absorbent resin prior to the modification. The degree of theshift, however, may vary and depends, for example, on the amount of aradical polymerization initiator mixed with the water absorbent resin,on the water content, on the conditions of irradiation with activeenergy rays, and on the flowing process during the irradiation.

The modified water absorbent resin which is obtained by the method ofthis invention has surface cross-links formed uniformly and with a highcross-link density throughout the entire surface of the water absorbentresin. Thereby good characteristics, such as absorption capacity,absorption speed, gel strength, and suction power which a waterabsorbent resin can be obtained. Conventionally, speed and extent of thesurface cross-linking have been found to depend on the ratio ofneutralization, when an acrylic acid type water absorbent resin issubjected to surface cross-linking by using such a surface cross-linkingagent as polyhydric alcohol, polyvalent epoxy compound, or alkylenecarbonate. Specifically, the surface cross-linking proceeds fast whenthe ratio of neutralization is low, while surface cross-linking proceedswith difficulties when the ratio of neutralization is high. For thepurpose of surface cross-linking the water absorbent resin to beobtained by the post-neutralization of polyacrylic acid,post-neutralization had to be performed uniformly after surfacecross-linking. Contrary thereto, according to this invention, surfacecross-linking of the water absorbent resin can be done independentlyfrom the ratio of neutralization of a water absorbent resin andindependently from uniformity of post-neutralization. It is believedthat since surface cross-linking depends on the action of a radicalpolymerization initiator on a main chain of the water absorbent resin,surface cross-linking can proceed regardless of whether a carboxyl groupis present in the form of an acid or a salt.

If this invention is executed in the presence of an ethylenicallyunsaturated monomer, the radical polymerization initiator is consumed bythe polymerization of the ethylenically unsaturated monomer, which isnot desirable in the present invention.

In accordance with this invention, surface treatment of the waterabsorbent resin can be carried out fully satisfactorily even at reactiontemperatures near room temperature, and the surface-treated waterabsorbent resin consequently obtained has good characteristics, such asabsorption capacity, absorption speed, gel strength, and suction powerwhich the water absorbent resin. Accordingly, the water absorbent resinwhich is obtained by this invention is optimally suitable for use inabsorbent members, such as disposable diapers, training pants, sanitarynapkins and other sanitary materials for absorbing body fluid.

Absorbent Articles

The absorbent members made by the method of the present invention arepreferably used as absorbent cores in absorbent articles. As usedherein, absorbent article refers to devices that absorb and containliquid, and more specifically, refers to devices that are placed againstor in proximity to the body of the wearer to absorb and contain thevarious exudates discharged from the body. Absorbent articles includebut are not limited to diapers, adult incontinent briefs, diaper holdersand liners, sanitary napkins and the like.

Preferred absorbent articles of the present invention are diapers andtraining pants. As used herein, “diaper” and “training pants” refers toan absorbent article generally worn by infants and incontinent personsabout the lower torso.

Absorbent articles especially suitable for the present inventiontypically comprise an outer covering including a liquid pervioustopsheet, a liquid impervious backsheet and an absorbent core generallydisposed between the topsheet and the backsheet. The absorbent core maycomprise any absorbent material that is generally compressible,conformable, non-irritating to the wearer's skin, and capable ofabsorbing and retaining liquids such as urine and other certain bodyexudates. In addition to the SAP particles of the present invention, theabsorbent core may comprise a wide variety of liquid-absorbent materialscommonly used in disposable diapers and other absorbent articles such ascomminuted wood pulp, which is generally referred to as air felt.

Exemplary absorbent structures for use as the absorbent assemblies aredescribed in U.S. Pat. No. 5,137,537 entitled “Absorbent StructureContaining Individualized, Polycarboxylic Acid Crosslinked Wood PulpCellulose Fibers” which issued to Herron et al. on Aug. 11, 1992; U.S.Pat. No. 5,147,345 entitled “High Efficiency Absorbent Articles ForIncontinence Management” issued to Young et al. on Sep. 15, 1992; U.S.Pat. No. 5,342,338 entitled “Disposable Absorbent Article ForLow-Viscosity Fecal Material” issued to Roe on Aug. 30, 1994; U.S. Pat.No. 5,260,345 entitled “Absorbent Foam Materials For Aqueous Body Fluidsand Absorbent Articles Containing Such Materials” issued to DesMarais etal. on Nov. 9, 1993; U.S. Pat. No. 5,387,207 entitled “Thin-Until-WetAbsorbent Foam Materials For Aqueous Body Fluids And Process For MakingSame” issued to Dyer et al. on Feb. 7, 1995; U.S. Pat. No. 5,397,316entitled “Slitted Absorbent Members For Aqueous Body Fluids Formed OfExpandable Absorbent Materials” issued to LaVon et al. on Mar. 14, 1995;and U.S. Pat. No. 5,625,222 entitled “Absorbent Foam Materials ForAqueous Fluids Made From high In al. on Jul. 22, 1997.

EXAMPLES

In the following, this invention will be described more specifically byworking examples and comparative examples. This invention is not limitedthereto. Hereinafter, the “parts by weight” may be expressed simply as“parts” and the “liters” simply as “L” for the sake of convenience. Themethod of determination and the method of evaluation indicated in theworking examples and the comparative example will be shown below.

(1) Particle Size Distribution: Weight Average Particle Diameter (D50)and Logarithmic Standard Deviation (σζ)

A water absorbent resin or particulate absorbent of 10 g is passedthrough JIS standard sieves having mesh sizes of 850 μm, 710 μm, 600 μm,500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, and 45 μm (THE IIDATESTING SIEVE, made by Lida Seisakusho K.K., 8 cm in diameter), at roomtemperature (20 to 25° C.) and at humidity of 50 RH %, and thenclassified by using a sieve shaker (IIDA SIEVE SHAKER, TYPE: ES-65type,SER. No. 0501, made by Iida Seisakusho K.K.) for 5 minutes. As for aweight average diameter, residual percentage R is plotted on alogarithmic probability paper, and from this plotting, a particlediameter corresponding to R=50 wt % reads as a weight average diameter(D50).

Further, the particle diameters with R being 84.1% by weight and 15.9%by weight are referred to as X1 and X2, respectively. The logarithmicstandard deviation (σζ) is represented by the following formula.Specifically, it means that the smaller the value σζ is, the narrowerthe particle size distribution is.σζ=0.5×ln(X2/X1)(2) Surface Water Content

500 g of a water absorbent resin as a base polymer are added to 5 litersof Loedige mixer (made by Loedige Co., Ltd., Type: M5R), and a treatingsolution obtained prior to mixing 5.0 g of ammonium persulfate, 2.5 g ofa monomethyl ether of a polyethylene glycol (a number average molecularweight of about 2,000) and 40 g of water, are sprayed thereto understirring at 300 rpm. After being mixed by stirring for 3 min at roomtemperature, the stirring is terminated. The resultant mixture of 1 g isadded to a screw tube, and 4 g of methanol anhydride is added. Then, themixture is shaken for 30 seconds with a mini-shaker MS1 made by IKAK.K., and thereafter it is absorbed with a syringe, then is filtratedwith a filter (made by Zeal Science Co., Ltd.; Water type 25 A (a porediameter of 0.45 μm)). The amount of water contained in a filtrate ismeasured by a method below with a Karl Fischer moisture meter (made byKyoto Electronics Manufacturing Co., Ltd.; Type: MKS-1S).

Measurement of amount of water with a Karl Fischer moisture meter

1. Principle for Measurement

This is a method of measuring an amount of water using a volumetricanalysis, wherein a Karl Fischer reagent in which water reactsquantitatively with iodine and sulfurous acid gas in the presence ofmethyl alcohol and pyridine, is used as a titrant.

Polarization is conducted by making slight constant electric currentbetween two platinum electrodes immersed in a solution, and an end pointof titration is determined by a Dead Stop method wherein a potentialchange caused by excessive iodine at an end point is detected. Tomeasure an amount of water by a Karl Fischer method, a sample is put ina flask for titration, titrated with a Karl Fischer reagent, and anamount of water in the sample is determined as a product of a titrationamount of a Karl Fischer reagent and a titer.W=K×Fwherein W is an amount of water (mg) in a sample;K is a titration amount of a Karl Fischer reagent (mL); andF is a titer of a Karl Fischer reagent (mg/mL).

2. Measuring Method

50 mL of a solvent for measurement (a mixed product of 50 mL of anacetic acid (special grade), 50 mL of Buffer solution (HYDRNAL-Buffer),and 900 mL of methanol anhydride) is charged until electrodes in a KarlFischer moisture meter are immersed therein. Then, titration isconducted with a Karl Fischer reagent by pushing a “START” key, to makean inner part of a flask for titration in an anhydrous state.

A sample is put into a flask for titration, titration is conducted witha Karl Fischer reagent by pushing a “START” key. A weight of the sample(a) [mg] and an amount of Karl Fischer titration (b) [mL] are recorded.Measuring was conducted by three times in all, and an average value iscalculated.

By inserting the weight of the sample (a) and the amount of Karl Fischertitration (b) into the equation (1) below, the water content (c) [wt %]in methanol anhydride, which is used in extraction of water from amixture containing a water absorbent resin, is calculated. As for F(titer of a Karl Fischer reagent), measuring is conducted by usingHYDRNAL-Composite 5K (about 5 mg H₂O/mL), and it is calculated byinserting the value into the equation (2) below.(c)=((b)×F/(a))×100  (1)F(mg/mL)=[HYDRNAL-Composite 5K (about 5 mg H₂O/mL)]×[a solution amountof HYDRNAL-Composite 5K [mL]]/[a titration amount of a Karl Fischerreagent [mL]]  (2)

The total (d) [wt %] water concentration contained in methanol anhydrideis measured. The water concentration derived from water contained in thewater absorbent resin before addition of the treating agent issubtracted from the water concentration (c) in methanol anhydride whichis used in extraction of water from the mixture containing the waterabsorbent resin mixture as calculated in the above described equation(1). Thereby, concentration (e), namely (c)−(d)=(e) is obtained. Theamount of water (g) [mg] which is extracted from the water absorbentresin mixture is calculated by using the concentration (e) and an amountof methanol anhydride (f) [mg] to be used in extraction of water fromthe water absorbent resin mixture, in accordance with the followingequation (3).(g)=((c)−(d))×(f)=(e)×(f)  (3)

Further, the amount of water (h) [mg] derived from the treatingsolution, which is contained in a water absorbent resin mixture (a), canbe calculated by using the following equation (4), based on the weight(i) [mg] of the treating solution added per 1,000 mg of the waterabsorbent resin and the weight of water (j) [mg] contained in thetreating solution.(h)=(a)×((j)/(1000+(i))  (4)

The ratio of water (g) extracted from the water absorbent resin to water(h) derived from the treating solution, which is contained in a waterabsorbent resin mixture (a), is calculated from the following equation(5), which is made as an extraction ratio (k) [wt %].(k)=((g)/(h))×100  (5)

The weight ratio (l) [wt %] of the amount of water contained in atreating solution added to the water absorbent resin multiplied with theamount of the water absorbent resin and the extraction ratio (k) givesthe surface water content (m) [wt %] according to the following equation(6):(m)=(l)×((k)/100)  (6)(3) Centrifuge Retention Capacity (CRC)

CRC indicates absorbency in an aqueous 0.90 wt. % sodium chloridesolution (hereinafter also called simply as “physiological saline”)without load for 30 min.

0.200 g of a water absorbent resin is uniformly put in a pouch (85 mm×60mm) made of a non-woven fabric (made by Nangoku Pulp Kogyo K.K., ProductName; Heatlon Paper, Model GSP-22), and heat-sealed. Then, the pouch isimmersed at room temperature in large excess (about 500 mL) ofphysiological saline. After 30 min, the pouch is pulled out, and wateris removed with a centrifuge (made by Kokusan Co., Ltd., Type: H-122) bycentrifugal force (250G) as described in “Edana ABSORBENCY II 441.1-99”for 3 min. Then, the weight of the pouch is measured, which is referredto as W1 (g). Further, the same is done without using the waterabsorbent resin, to measure the weight, which is referred to as W0 (g).From these values, W1 and W0, the CRC (g/g) is calculated according tothe equation below.

CRC  (g/g) = [(W 1 − W 0)/Weight  of  water  absorbent  resin] − 1(4) Absorbency Against Pressure (AAP)

400-mesh wire gauze of stainless steel (38 μm in mesh size) is welded toa bottom of a plastic supporting cylinder having an inner diameter of 60mm. At room temperature (25±2° C.) and 50 RH% of humidity, 0.900 g of agiven water absorbent resin is uniformly scattered on the wire gauze. Apiston and a load, each of which is adjusted to exert a load of 4.83 kPauniformly on the water absorbent resin, has an outer diameter slightlysmaller than 60 mm but produces no gap relative to the inner wallsurface of the supporting cylinder, and does not have its unobstructedvertical motion prevented, were mounted thereon sequentially in theorder mentioned, and the whole weight W_(a) (g) of the resultantmeasuring device is determined

A glass filter 90 mm in diameter (pore diameter: 100 to 120 μm: made bySogo Rikagaku Glass Manufactory K.K.) is placed inside a petri dish 150mm in diameter. An aqueous 0.9 wt. % sodium chloride solution(physiological saline) (20-25° C.) is added to the petri dish so as togive the same level as the upper surface of the glass filter. One filterpaper 90 mm in diameter (0.26 mm in thickness and 5 μm in retainedparticle diameter; made by Advantec Toyo K.K. and sold under the productname of “JIS P 3801, No. 2”) is mounted on the surface of physiologicalsaline so as to have the surface thereof thoroughly wetted and theexcess solution is removed.

The resultant measuring device is wholly mounted on the wetted filterpaper and the water absorbent resin is allowed to absorb the solutionunder load for a prescribed time of one hour. The whole measuring deviceis lifted after the one hour's standing, and the weight thereof W_(b)(g) is determined This determination of the weight must be performed asquickly as possible without exposing the device to any vibration. Theabsorbency against pressure (AAP) (g/g) is calculated in accordance withthe following formula using W_(a) and W_(b).

AAP  (g/g) = [W_(b)  (g) − W_(a)  (g)]/Weight  of  water  absorbent  resin  (g)(5) Total Water Content

In an aluminum cup having a bottom with a diameter of 4 cm and a heightof 2 cm, 1.00 g of a water absorbent resin is spread uniformly on thebottom. The aluminum cup containing the water absorbent resin is weighed[W4 (g)]. The cup is left in a hot air drier kept at 180° C. for 3 hoursImmediately (within at least 1 minute) after the cup is taken out of thehot air drier, the aluminum cup containing the water absorbent resin isweighed [W5 (g)]. The total water content is calculated from the valuesW4 and W5 by the following formula.

Total  water  content  (%  by  weight)=    [(W 4  (g) − W 5  (g))/(Weight  of  water  absorbent  resin  (g))] × 100(6) Saline Flow Conductivity (SFC)

SFC is a value which indicates the degree of liquid permeabilityexhibited by water absorbent resin particles in a swollen state. Alarger SFC value indicates higher liquid permeability.

The SFC is determined in accordance with the test for the saline flowconductivity (SFC) described in JP-T-9 (1997)-509591 with necessarymodification.

Specifically, by the use of a device illustrated in the FIGURE, SFC isdetermined In the device illustrated in FIG. 1, a tank 31 has a glasstube 32 inserted therein and the lower end of the glass tube 32 isdisposed so that an aqueous 0.69 wt. % sodium chloride solution 33 canbe maintained to a height of 5 cm from the bottom of the swelled gel 44held in a cell 41. The aqueous 0.69 wt. % sodium chloride solution inthe tank 31 is supplied to the cell 41 via an L-letter tube 34 fittedwith a cock. Below the cell 41, a container 48 for collecting the passedliquid is disposed and the collecting container 48 is set on a pan scale49. The cell 41 has an inside diameter of 6 cm. A wire gauze (opening ofsieve: 38 μm) 42 of stainless steel of No. 400 is disposed on the bottomsurface in the lower part of the cell. A piston 46 is provided in thelower part thereof with holes 47 sufficient for passing a liquid, andfitted in the bottom part thereof with a glass filter 45 having goodpermeability so as to prevent the water absorbent resin or the swelledgel thereof from entering the hole 47. The cell 41 is laid on a standfor mounting the cell. The surface of the stand contacting the cell isplaced on a wire gauze 43 of stainless steel incapable of obstructingthe passage of liquid.

Artificial urine is prepared by mixing 0.25 g of calcium chloridedihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloridehexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogenphosphate, 0.15 g of diammonium hydrogen phosphate, and 994.25 g ofpurified water together.

Water absorbent resin (0.900 g) is uniformly placed in a container 40and left swelling with an artificial urine under a pressure of 0.3 psi(2.07 kPa) for 60 minutes, and a height of a gel layer of gel 44 isrecorded. Subsequently, under a pressure of 0.3 psi (2.07 kPa), anaqueous 0.69 wt. % sodium chloride solution 33 from a tank 31 is passedunder a stated hydrostatic pressure through the swelled gel layer. Bymeans of a computer and a balance, the amounts of liquid passing throughthe gel layer at intervals of 20 seconds are recorded as a function oftime over 10 minutes. A flow speed Fs (t) through the swelled gel 44(mainly between adjacent particles) is determined in unit of [g/s] bydividing an increased weight (g) by an increased time (second). The timein which the constant hydrostatic pressure and the stable flow speed areattained is denoted as Ts. The data obtained for 10 minutes and Ts areexclusively used for the calculation of flow speed. The value Fs (t=0),namely an initial flow speed through the gel layer, is calculated byusing the flow speed obtained over 10 minutes and Ts. Specifically, theFs (t=0) is calculated by extrapolating the result of the least-squaresmethod performed on the Fs (t) against time into t=0.

$\begin{matrix}{{SFC} = {\lbrack {{{Fs}( {t = 0} )} \times L\; 0} \rbrack/( {\rho \times A \times \Delta\; P} )}} \\{= {\lbrack {{{Fs}( {t = 0} )} \times L\; 0} \rbrack/139506}}\end{matrix}$wherein Fs (t) stands for a flow speed expressed in units of [g/s],L0 stands for a height of a gel layer expressed in units of cm,ρ stands for a density of an aqueous 0.69 wt. % sodium chloride solution(1.003 g/cm³),A stands for an upper side area of a gel layer in a cell 41 (28.27 cm²),ΔP stands for a hydrostatic pressure exerted on a gel layer (4920dynes/cm²), anda unit of SFC is (10⁻⁷·cm³·s·g⁻¹).

Production Example 1

In a reaction vessel which is formed from a jacketed, double-arm typekneader of stainless steel with an inner volume of 10 L and providedwith two sigma-type blades, and a lid further attached thereto, 5,437 gof an aqueous solution of sodium acrylate (a monomer concentration: 39wt. %) having a neutralization ratio of 60 mol % is placed. Then, 7.90 gof a polyethylene glycol diacrylate (a number of average ethylene oxideunits: n=9) as an internal cross-linking agent is dissolved in theaqueous solution, to prepare a reaction solution. Further, the reactionsolution is deaerated under a nitrogen atmosphere. Subsequently, 30.19 gof an aqueous 10 wt. % sodium persulfate solution as a polymerizationinitiator and 25.16 g of an aqueous 0.1 wt. % L-ascorbic acid solutionare added to the reaction solution while stirring. As a result,polymerization begins after about one minute. While pulverizing the gelformed, polymerization is conducted at 20 to 95° C., and thehydrogel-like cross-linked polymer is taken out 30 minutes after thebeginning of the polymerization. The particle diameter of thehydrogel-like cross-linked polymer obtained is not larger than 5 mm. Thepulverized hydrogel-like cross-linked polymers are scattered on a wiremesh of 50 mesh (opening of sieve: 300 μm), and are dried in hot air at175° C. for 50 minutes. Thus, easily pulverizable powdery agglomerateshaving an amorphous form are obtained.

The resultant powdery agglomerates are pulverized with a roll mill, andare further classified with a JIS standard sieve having an opening ofsieve of 710 μm. Next, particles which passed through a sieve having anopening of sieve of 710 μm in the above-described operation, areclassified with a JIS standard sieve having a opening of sieve of 150μm, to remove particles which pass through a sieve having a opening ofsieve of 150 μm. Thus, water absorbent resin (A) is obtained.

The particle distribution of the resultant water absorbent resin (A) isshown in Table 1 below, and various properties thereof are shown inTable 2 below.

Production Example 2

In a reaction vessel which is formed from a jacketed, double-arm typekneader of stainless steel with an inner volume of 10 L and providedwith two sigma-type blades, and a lid further attached thereto, 5,443 gof an aqueous solution of sodium acrylate (a monomer concentration: 39wt. %) having a neutralization ratio of 90 mol % is placed. Then, 6.11 gof a polyethylene glycol diacrylate (a number of average ethylene oxideunits: n=9) as an internal cross-linking agent is dissolved into theaqueous solution, to prepare the reaction solution. Further, thereaction solution is deaerated under nitrogen atmosphere. Subsequently,28.02 g of an aqueous 10 wt. % sodium persulfate solution as apolymerization initiator and 23.35 g of an aqueous 0.1 wt. % L-ascorbicacid solution are added to the reaction solution while stirring. As aresult, polymerization begins after about one minute. While pulverizingthe gel formed, polymerization is conducted at 20 to 95° C., and thehydrogel-like cross-linked polymer is taken out 30 minutes after thebeginning of the polymerization. The particle diameter of thehydrogel-like cross-linked polymer obtained is not larger than 5 mm. Thepulverized hydrogel-like cross-linked polymers are scattered on a wiremesh of 50 mesh (opening of sieve: 300 μm), and are dried in hot air at175° C. for 50 minutes. Thus, easily pulverizable powdery agglomerateshaving an amorphous form are obtained.

The resultant powdery agglomerates are pulverized with a roll mill, andare further classified with a JIS standard sieve having an opening ofsieve of 710 μm. Next, particles which passed through a sieve having anopening of sieve of 710 μm in the above-described operation, areclassified with a JIS standard sieve having a opening of sieve of 150μm, to remove particles which pass through a sieve having a opening ofsieve of 150 μm. Thus, water absorbent resin (B) is obtained.

The particle distribution of the resultant water absorbent resin (B) isshown in Table 1 below, and various properties thereof are shown inTable 2 below. In Table 1, “not less than 850 μm” is referred to as theratio (% by weight) of the water absorbent resin which remains on thesieve having a mesh size of 850 μm following the classification process.Also, “not more than 45 μm” is referred to as the ratio (% by weight) ofthe water absorbent resin which passes through a sieve having a meshsize of 45 μm following the classification process. Then, “x to y isreferred to as the ratio (% by weight) of the water absorbent resinwhich passes through a sieve having a mesh size of x μm and also remainson a sieve having a mesh size of y μm following the classificationprocess.

TABLE 1 Production Example 1 2 Water absorbent resin A B D50 (μm) 345345 σζ 0.327 0.327 Particle size distribution not less than 850 μm (wt%) 0.0 0.0 850 to 710 μm (wt %) 0.1 0.1 710 to 600 μm (wt %) 1.0 1.0 600to 500 μm (wt %) 3.7 3.7 500 to 425 μm (wt %) 21.7 21.7 425 to 300 μm(wt %) 39.6 39.6 300 to 212 μm (wt %) 23.0 23.0 212 to 150 μm (wt %) 9.39.3 150 to 45 μm (wt %) 1.5 1.5 not more than 45 μm (wt %) 0.1 0.1 Total(wt %) 100.0 100.0

Example 1

500 g of the water absorbent resin (A) as a base polymer are added to 5L of Loedige mixer (made by Loedige Co., Ltd., Type: M5R). A treatingsolution which had been prepared by mixing 12.5 g of ammoniumpersulfate, 2.5 g of polyethylene glycol monomethyl ether (a numberaverage molecular weight of about 2,000) and 120 g of water, is sprayedunder stirring at 300 rpm. After mixing is continued under stirring foradditional 3 minutes at room temperature, to achieve permeation anddiffusion of the added water into the inner part of particles, stirringis terminated once, and a sample charging port of a proshear mixer istaken out. the surface water content of the water absorbent resincomposition (1) thus obtained is 10.4% by weight.

After putting a glass plate made of quartz and having a thickness of 3mm at opening part, stirring of the water absorbent resin composition(1) is restarted (a time necessary for restart was 30 seconds). Aradiation device able to emit ultraviolet rays (made by Ushio DenkiK.K., UV-152/IMNSC3-AA06) furnished with a metal halide lamp of 1 kW(made by the same company, UVL-1500M2-N1) is set at a distance of 8 cmbetween a center of the lamp and a quartz plate. Then, the waterabsorbent resin composition (1) is irradiated with ultraviolet rays atroom temperature for 15 minutes, to obtain the modified water absorbentresin (1).

The thus obtained modified water absorbent resin (1) is tested forvarious properties, and the results are shown in Table 2 below. In theTable 2, “CRC after correction with total water content” and “AAP aftercorrection with total water content” are calculated by the followingformulas. In the following formulas, “CRC before correction with totalwater content” is referred to as a centrifuge retention capacity (CRC)of water absorbent resin prior to determination of total water contentby the formula (5), and “AAP before correction with total water content”is referred to as an absorbency against pressure (AAP) of waterabsorbent resin prior to determination of total water content by theformula (5).CRC after correction with total water content (g/g)=[(CRC beforecorrection with total water content) (g/g)+1]/(100−Total water contentof water absorbent resin)]×100-1AAP after correction with total water content (g/g)=AAP beforecorrection with total water content (g/g)/(100−Total water content ofwater absorbent resin)]×100

Example 2

The same procedure as described Example 1 is repeated except that thewater amount in the treating solution is changed to 160 g, to obtainwater absorbent resin composition (2) having a surface water content of12.5% by weight. Further, by the same procedure as described Example 1,the water absorbent resin composition (2) is irradiated with ultravioletrays for 15 minutes, to obtain modified water absorbent resin (2).

The thus obtained modified water absorbent resin (2) is tested forvarious properties, and the results are shown in Table 2 below.

Example 3

The same procedure as described Example 1 is repeated except that thewater amount in the treating solution is changed to 200 g, to obtain awater absorbent resin composition (3) having a surface water content of15.5% by weight. Further, by the same procedure as described Example 1,the water absorbent resin composition (3) is irradiated with ultravioletrays for 15 minutes, to obtain the modified water absorbent resin (3).

The thus obtained modified water absorbent resin (3) is tested forvarious properties, and the results are shown in Table 2 below.

Example 4

A water absorbent resin composition (4) having a surface water contentof 12.5% by weight is obtained by repeating the same procedure asdescribed Example 2. Further, by the same procedure as described Example1, the water absorbent resin composition (4) is irradiated withultraviolet rays for 1 minute, to obtain the modified water absorbentresin (4).

The thus obtained modified water absorbent resin (4) is tested forvarious properties, and the results are shown in Table 2 below.

Example 5

The same procedure as described Example 1 is repeated except that thewater absorbent resin (B) is used as a base polymer instead, to obtain awater absorbent resin composition (5) having a surface water content of6.5% by weight. Further, by the same procedure as described Example 1,the water absorbent resin composition (5) is irradiated with ultravioletrays for 15 minutes, to obtain the modified water absorbent resin (5).

The thus obtained modified water absorbent resin (5) is tested forvarious properties, and the results are shown in Table 2 below.

Example 6

The same procedure as described Example 2 is repeated except that thewater absorbent resin (B) is used as a base polymer instead, to obtain awater absorbent resin composition (6) having a surface water content of6.7% by weight. Further, by the same procedure as described Example 1,the water absorbent resin composition (6) is irradiated with ultravioletrays for 15 minutes, to obtain the modified water absorbent resin (6).

The thus obtained modified water absorbent resin (6) is tested forvarious properties, and the results are shown in Table 2 below.

Example 7

The same procedure as described Example 3 is repeated except that thewater absorbent resin (B) is used as a base polymer instead, to obtainthe water absorbent resin composition (7) having a surface water contentof 9.6% by weight. Further, by the same procedure as described Example1, the water absorbent resin composition (7) is irradiated withultraviolet rays for 15 minutes, to obtain the modified water absorbentresin (7).

The thus obtained modified water absorbent resin (7) is tested forvarious properties, and the results are shown in Table 2 below.

Control 1

The same procedure as described Example 1 is repeated except that thewater amount in the treating solution is changed to 20 g, to obtain awater absorbent resin composition for comparison (1) having a surfacewater content of 1.9% by weight. Further, by the same procedure asdescribed Example 1, the water absorbent resin composition forcomparison (1) is irradiated with ultraviolet rays for 15 minutes, toobtain the modified water absorbent resin for comparison (1).

The thus obtained modified water absorbent resin for comparison (1) istested for various properties, and the results are shown in Table 2below.

Control 2

The same procedure as described Example 1 is repeated except that thewater amount in the treating solution is changed to 40 g, to obtain thewater absorbent resin composition for comparison (2) having a surfacewater content of 4.5% by weight. Further, by the same procedure asdescribed Example 1, the water absorbent resin composition forcomparison (2) is irradiated with ultraviolet rays for 15 minutes, toobtain the modified water absorbent resin for comparison (2).

The thus obtained modified water absorbent resin for comparison (2) istested for various properties, and the results are shown in Table 2below.

Control 3

The same procedure as described Example 1 is repeated except that thewater amount in the treating solution is changed to 80 g, to obtain thewater absorbent resin composition for comparison (3) having a surfacewater content of 7.9% by weight. Further, by the same procedure asdescribed Example 1, the water absorbent resin composition forcomparison (3) is irradiated with ultraviolet rays for 15 minutes, toobtain the modified water absorbent resin for comparison (3).

The thus obtained modified water absorbent resin for comparison (3) istested for various properties, and the results are shown in Table 2below.

Control 4

A water absorbent resin composition for comparison (4) having a surfacewater content of 4.5% by weight is obtained by repeating the sameprocedure as described Control 2. Further, by the same procedure asdescribed Example 4, the water absorbent resin composition forcomparison (4) is irradiated with ultraviolet rays for 1 minute, toobtain the modified water absorbent resin for comparison (4).

The thus obtained modified water absorbent resin for comparison (4) istested for various properties, and the results are shown in Table 2below.

Control 5

The same procedure as described Control 1 is repeated except that thewater absorbent resin (B) is used as a base polymer instead, to obtainthe water absorbent resin composition for comparison (5) having asurface water content of 1.1% by weight. Further, by the same procedureas described Example 1, the water absorbent resin composition forcomparison (5) is irradiated with ultraviolet rays for 15 minutes, toobtain the modified water absorbent resin for comparison (5).

The thus obtained modified water absorbent resin for comparison forcomparison (5) is tested for various properties, and the results areshown in Table 2 below.

Control 6

The same procedure as described Control 3 is repeated except that thewater absorbent resin (B) is used as a base polymer instead, to obtainthe water absorbent resin composition for comparison (6) having asurface water content of 5.2% by weight. Further, by the same procedureas described Example 1, the water absorbent resin composition forcomparison (6) is irradiated with ultraviolet rays for 15 minutes, toobtain the modified water absorbent resin for comparison (6).

The thus obtained modified water absorbent resin for comparison forcomparison (6) is tested for various properties, and the results areshown in Table 2 below.

TABLE 2 Mixing time Irradiation Surface Total Total water content Totalwater content Composition of of treating time water water beforecorrection after correction SFC treating solution solution of UV contentcontent CRC AAP CRC AAP (10⁻⁷ · (wt. %)) (min.) (min.) (wt. %) (wt. %)(g/g) (g/g) (g/g) (g/g) cm³ · s · g⁻¹) Production WAR (A) — — — 0.1 5.436.7 6.6 38.9  7.0 0 Ex. 1 Example 1 WAR (1) APS/PEG-OMe/W = 2.5/0.5/2410 15 10.4 18.2 15.7 16.2

258 Example 2 WAR (2) APS/PEG-OMe/W = 2.5/0.5/32 10 15 12.5 25.4 12.913.5

360 Example 3 WAR (3) APS/PEG-OMe/W = 2.5/0.5/40 10 15 15.5 28.4 12.212.9

427 Example 4 WAR (4) APS/PEG-OMe/W = 2.5/0.5/32 10 1 12.5 28.1 18.815.6

74 Control 1 WAR for com. (1) APS/PEG-OMe/W = 2.5/0.5/4 10 15 1.9 8.628.7 21.8

15 Control 2 WAR for com. (2) APS/PEG-OMe/W = 2.5/0.5/8 10 15 4.5 10.225.3 22.1

61 Control 3 WAR for com. (3) APS/PEG-OMe/W = 2.5/0.5/16 10 15 7.9 15.218.9 18.2

159 Control 4 WAR for com. (4) APS/PEG-OMe/W = 2.5/0.5/8 10 1 4.5 12.226.8 16.5

8 Production Ex. 2 WAR (B) — — — 0.1 5.2 34.6 5.9 36.6  6.2 0 Example 5WAR (6) APS/PEG-OMe/W = 2.5/0.5/24 10 15 6.5 24.3 16.1 16.3

163 Example 6 WAR (7) APS/PEG- OMe/W = 2.5/0.5/32 10 15 6.7 30.0 12.613.8

340 Example 7 WAR (8) APS/PEG-OMe/W = 2.5/0.5/40 10 15 9.6 32.4 11.612.7

417 Control 5 WAR for com. (5) APS/PEG-OMe/W = 2.5/0.5/4 10 15 1.1 12.830.0 7.0 34.6  8.0 0 Control 6 WAR for com. (6) APS/PEG-OMe/W =2.5/0.5/16 10 15 5.2 20.0 19.4 17.9 24.5 22.4 91 WAR: Water absorbentresin, WAR for com.: Water absorbent resin for comparison APS: Ammoniumpersulfate PEG-OMe: Polyethylene glycol monomethyl ether (a numberaverage molecular weight of about 2,000) W: Pure water

It is noted from the results shown in Table 1 that according to themethod for the production of this invention, by irradiating the waterabsorbent resin composition with active energy rays, water and awater-soluble radical polymerization initiator with a surface watercontent of the water absorbent resin being controlled to a level of notlower than a predetermined value, the modification of the surface of thewater absorbent resin particle can be effectively conducted, to producea water absorbent resin having excellent water absorbent properties.Further, it is also noted from the results of Example 4 that byirradiating water absorbent resin with active energy rays whilecontrolling the surface water content of the water absorbent resin to alevel of not less than a prescribed value by adding relatively largeamount of water thereto, the modification of surface of water absorbentresin particles can be efficiently carried out even in a short time anda water absorbent resin having excellent water absorbent properties canbe produced.

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm” is intended to mean“about 40 mm”

All documents cited in the Detailed Description of the Invention are, inrelevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention. To the extent that any meaning ordefinition of a term in this written document conflicts with any meaningor definition of the term in a document incorporated by reference, themeaning or definition assigned to the term in this written documentshall govern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. A disposable absorbent article, comprising: (a) a topsheet; (b) abacksheet; (c) an absorbent member disposed between the topsheet andbacksheet, the absorbent member including a water absorbent resin in theform of a plurality of water-swellable, water-insoluble, cross-linkedpolymer particles, at least some of the polymer particles including asubstantially uniform structure of surface cross-links formed by i.mixing the polymer particles, water, and a radical polymerizationinitiator, without addition of an ethylenically unsaturated monomer, toobtain a water absorbent resin composition, the water being present inan amount that exceeds 20 parts by weight but is not more than 100 partsby weight based on 100 parts by weight of the water absorbent resin, ii.irradiating said water absorbent resin composition with active energyrays to provide the surface cross-link structure, wherein the surfacewater content of the polymer particles is not lower than 3.0% accordingto the equation: (m)=(l)×((k)/100), wherein m is the surface watercontent in wt %, l is the weight ratio of the amount of water containedin a treating solution in wt %, and k is the extraction ratio in wt %;and (d) wherein the water absorbent resin exhibits an absorbency ofphysiological saline against pressure of 4.83 kPa of between about 8 g/gand about 40 g/g, when measured according to the Absorbency AgainstPressure test herein.
 2. The disposable absorbent article of claim 1,wherein the radical polymerization initiator is at least one ofwater-soluble and heat-degradable.
 3. The disposable absorbent articleof claim 2, wherein the radical polymerization initiator is selectedfrom the group consisting of persulfates, hydrogen peroxide andwater-soluble azo compounds.
 4. The disposable absorbent article ofclaim 1, wherein said radical polymerization initiator is present in anamount of between about 0.01 and about 20 parts by weight, based on 100parts by weight of the water absorbent resin.
 5. The disposableabsorbent article of claim 1, wherein said radical polymerizationinitiator is in the form of an aqueous solution.
 6. The disposableabsorbent article of claim 1, wherein said water absorbent resincontains an acid group and has a neutralization ratio (mol % of theneutralized acids group in the whole of acid groups) in the range offrom about 50 to about 75 mol %.
 7. The disposable absorbent article ofclaim 1, wherein said active energy rays are ultraviolet rays.
 8. Thedisposable absorbent article of claim 1, wherein said water absorbentresin is a powder and includes a polymer of acrylic acid (salt) as amain component.
 9. The disposable absorbent article of claim 1, whereinsaid water absorbent resin particles have a particle diameter in therange of from about 150 μm to about 850 μm.
 10. The disposable absorbentarticle of claim 1, wherein the saline flow conductivity of the waterabsorbent resin is not less than about 100×10⁻⁷·cm³·s·g⁻¹.
 11. Thedisposable absorbent article of claim 1, wherein the saline flowconductivity of the water absorbent resin is not less than about50×10⁻⁷·cm³·s·g⁻¹.
 12. The disposable absorbent article of claim 1,wherein said water absorbent resin has a total water content of lessthan about 50% by weight based on the weight of the water absorbentresin.
 13. The disposable absorbent article of claim 1, wherein thewater absorbent resin has a centrifuge retention capacity of betweenabout 10 and about 50 when measured according to the CentrifugeRetention Capacity method herein.
 14. The disposable absorbent articleof claim 1, wherein the water absorbent resin exhibits an absorbency ofphysiological saline against pressure of 4.83 kPa of at least about 22g/g.
 15. The disposable absorbent article of claim 1, the absorbencyagainst pressure of the water absorbent resin is improved by at least 1g/g when compared with the absorption against pressure of the resinprior to the modification.
 16. The disposable absorbent article of claim1, wherein the water absorbent resin includes an agent for enhancingliquid permeability, the agent being present in an amount of betweenabout 0.1 and about 20 parts by weight based on 100 parts by weight ofthe water absorbent resin.
 17. The disposable absorbent article of claim16, wherein the agent is selected from the group consisting of talc;kaolin; Fuller's earth; bentonite; activated clay; barite; naturalasphaltum; strontium ore; ilmenite; and pearlite; aluminum sulfates14-18 hydrates or anhydrides; potassium aluminum sulfates 12 hydrate;sodium aluminum sulfate 12 hydrate; aluminum chloride; aluminumpolychloride; and aluminum oxide; and aqueous solutions thereof; otherpolyvalent metal salts; hydrophilic amorphous silica; siliconoxide-aluminum; oxide-magnesium; silicon oxide-aluminum oxide composite,and silicon oxide-magnesium oxide composite.
 18. The disposableabsorbent article of claim 1, wherein the water absorbent resin has aresidual monomer content of less than 200 parts per million.
 19. Thedisposable absorbent article of claim 1, wherein the article is adiaper, adult incontinent brief, diaper holder, diaper liner, orsanitary napkin.